10.5.5 is about to come out.  I tried to find some soft statistics on the number of bug fixes in each of the point updates:

30 bug fixes 10.5.1
100 bug fixes 10.5.2
200 bug fixes 10.5.3
50 bug fixes 10.5.4
125 bug fixes 10.5.5
====================
500 bug fixes?

<a href="http://www.appleinsider.com/articles/08/06/30/apple_releases_mac_os_x_10_5_4_update.html" target="_blank">10.5.4</a>

The full list of fixes in Mac OS X 10.5.4 follows below:

General

* Includes recent Apple security updates.
* Resolves an issue with saving and reopening Adobe Creative Suite 3 files on a remote server.
* Includes additional RAW image support for several cameras.
* Addresses an issue that may result in a partially installed X11 application.
* Improves L2TP VPN client reliability.

AirPort

* Addresses AirPort reliability issues with 5GHz networks.
* Addresses AirPort issues that may result in slower performance in Logic Studio or MainStage.

iCal

* Improves overall iCal reliability for meeting requests, cancellation notices, delegation, and syncing with iPhone.
* Resolves an issue that prevents deleting an iCal event without notifying the creator.
* Addresses an issue in which events in all calendars affect availability. A checkbox now enables information-only calendars to be transparent from free/busy lookups.
* Resolves a UI issue preventing delegated calendars from showing up as a separate window.
* Addresses an issue with copying and pasting attendees from one event to another.
* Resolves an issue in which iCal may not delete events after a specified time interval, even when set to do so in iCal preferences.
* Addresses an issue in which To Dos cannot be marked private.

Safari

* Addresses a potential performance issue when loading secure web pages.
* Resolves issues that may be encountered when accessing secure web pages with client certificates that reside on a smart card.

Spaces and Exposé

* Addresses an issue in which switching from a space with a Finder window keeps the Finder as the active application instead of the application residing in the destination space.
* Fixes an issue in which dragging an application from the list of application assignments in Spaces System Preferences does not assign the application to the desired space.
* Resolves an Exposé issue that may result in only a subset of windows being shown.

<a href="http://www.appleinsider.com/articles/08/05/28/apple_releases_mac_os_x_10_5_3_update.html" target="_blank">10.5.3</a>

General

* Fixes a font issue that could result in Helvetica Narrow being used in applications instead of Helvetica.
* Addresses an issue with stuttering video and audio playback in certain USB devices.
* Resolves stability issues with Word of the Day, iTunes Artwork, and Slideshow screen savers.
* Fixes an issue in which certain attached hard drives may not show up in the Finder.
* Addresses an issue with .Mac syncing of Dashboard widgets over multiple Macs that use different screen resolutions.
* Includes additional RAW image support for several cameras.
* Improves the accuracy of the Software Update progress bar indicator.
* Addresses an issue in which Finder may not be available if the computer name is blank in Sharing preferences.
* Improves Active Directory binding and login.
* Eliminates a delay when logging in as an Active Directory user in a .local domain.
* Improves Spotlight searches on a AFP file server volumes.
* Clients can now change their password at the login window when bound to a Mac OS X 10.4 Open Directory server.
* Improves Safari reliability when connecting to the Internet through a Microsoft ISA proxy.

Address Book

* Addresses reliability issues when searching for contacts using built-in search.
* Resolves issues with mapping addresses that contain an ampersand character (&amp;).

AirPort

* Improves 802.1X behavior and reliability.
* Improves reliability when using Time Capsule.

Automator

* Addresses an issue in which some actions may not work with the "Show When Run" option enabled.
* Resolves an issue in which the "New iCal Event" action may not work.
* Resolves an issue that prevents workflows from being saved in the Finder's contextual menu.
* Fixes reliability issues for Automator scripts that search for files by date.
* Resolves an issue that prevents workflows from being saved in the Finder's contextual menu.
* Addresses an issue in which Automator workflows as Finder plugins do not work when the workflow begins with the "Get Selected Finder Items" action.
* Fixes an issue in which the "Copy Files" action does not reliably work when added from Automator’s warning dialog.

iCal

* Addresses potential privacy issues by allowing events to be marked as private.
* Resolves an issue in which the inspector does not show capacity and availability info for conference rooms within a building.
* Addresses an issue in which the current day could appear in the left-most column of the weekly view.
* Addresses reliability issues with meeting alarms, invitations and attachments.
* Resolves issues with reliability when restoring from iCal backups.
* Fixes accuracy issues with auto-completion, availability data and location names.
* Resolves an issue in which iCal may send cancellation notices for events in the past after a calendar is deleted.
* Fixes reliability issues with iCal syncing.

iChat

* Addresses reliability issues with screen sharing.
* Resolves an issue in which saved chat transcripts may reported as "still in use" after opening and closing them in iChat.
* Resolves an issue with group chats not being indexed in Spotlight.
* Only the last 250 messages of an active chat are saved.  Fixed to save unlimited number of lines.
* Addresses issues with echo cancellation that may occur on portable Macs.

Mail

* Resolves an issue in which Mail may prevent idle sleep when set to automatically check for new messages every minute.
* Addresses stability issues that may be encountered when dragging large attachments into an email message.
* Fixes an issue that could occur if two compose windows are open when dragging a file to the Mail icon in the Dock.
* Addresses reliability issues when changes are made to a mailbox while offline.
* Resolves wrapping issues that may be found with consecutive spaces in plain text.
* Fixes issues with certain web pages appearing garbled when emailed from Safari.
* Fixes an issue in which the Sent, Drafts, and Outbox mailboxes incorrectly list the "cc" recipients in the "To" column.
* Addresses reliability issues with attachments added to plain text notes.
* Fixes reliability issues with authenticated RSS feeds.
* Resolves an issue in which attaching an alias to an email message may not send the actual file.

Parental Controls

* Addresses reliability issues with application logging and time limits.
* Resolves an issue in which Parental Controls may prevent forced sleep.
* Addresses performance issues with web content filters.
* Fixes an issue with managed accounts in which iChat transcripts may not be created.
* Addresses issues with 4-byte files and whitelist.

Spaces

* Resolves an issue in which switching to a different space and returning back to the original space may reorder the application windows with a different active window.
* Resolves an issue in which activating an application from the Dock switches to a different space, even if there is a window for that application in the current space.
* Fixes an issue in which Command-Tab may incorrectly switch to a new space.
* Addresses reliability issues with Spaces when syncing preferences over .Mac.

Time Machine

* Includes fixes for Time Machine compatibility with Time Capsule.
* Resolves certain issues when backing up a portable Mac that is on battery power.
* Addresses compatibility issues with Aperture 2.
* Addresses reliability issues when performing a full restore from a Time Machine backup.
* Fixes an issue in which certain function keys may be disabled after using Time Machine.
* Fixes a possible alert message that incorrectly states a backup volume does not have enough space.
* Updates Time Machine to reliably restore attachments and messages in Mail.

VoiceOver

* Includes Braille Update 1.0 which enables GW Micro, HandyTech, HIMS, Nippon, and Papenmeier Refreshable Braille displays.
* Addresses an issue with Braille dot 7 and 8 underlining.
* Fixes an issue in which HTML page anchors may be ignored by the VoiceOver cursor.
* Fixes an issue that prevented Hot Spots from being used in text areas.
* Resolves an issue with spell checking in which VoiceOver may only announce the first misspelled word if there are multiple words spelled incorrectly.

<strong><a href="http://www.appleinsider.com/articles/08/02/11/apple_releases_mac_os_x_10_5_2_update.html" target="_blank">10.5.2</a></strong>

Active Directory

* Addresses issues which could hinder or prevent binding Mac OS X 10.5.x clients to Active Directory domains.

AirPort

* Improves connection reliability and stability
* Includes 802.1X improvements
* Resolves certain kernel panics

Back to my Mac

* Adds support for more third-party routers, as detailed in this article

Dashboard

* Improves performance of certain Apple Dashboard widgets (such as Dictionary)
* Addresses an issue in which Dashboard widgets may no longer be accessible after switching to or from an account that has Parental Controls enabled

Dock

* Updates Stacks with a List view option, a Folder view option, and an updated background for Grid view

Desktop

* Addresses legibility issues with the menu bar with an option to turn off transparency in Desktop &amp; Screen Saver preferences
* Adjusts menus to be slightly-less translucent overall

iCal

* Improves iCal so that it accurately reflects responses to recurring meetings
* Addresses an issue in which a meeting may remain on the calendar after being cancelled
* Addresses stability issues related to .Mac syncing of iCal calendars
* Resolves an intermittent issue in which editing an event with attendees would cause the event to shrink and not register that the event was updated

iChat

* Addresses an issue with simultaneously-logged in accounts in which iChat sounds generated from one account might be heard in another account
* Fixes an issue in which iChat idle time is affected by Time Machine backups
* Improves connectivity when running iChat behind a router that doesn’t preserve ports
* Enables logged chats from previous versions of iChat to open faster and more reliably
* Addresses an issue with text chats in which users may be unable to receive messages from the sender
* Addresses an issue that may prevent rejoining an AIM chat room without reopening iChat
* Addresses video chat compatibility issues with AIM 6 and third-party routers
* Fixes an issue with case-sensitivity of AIM handles

iSync

* Adds support for Samsung D600E and D900i phones

Finder

* Addresses an issue in which Finder could unexpectedly quit when displaying folder contents in Column view
* Addresses an issue in which Finder could unexpectedly quit when accessing Users and Groups in a Get Info pane
* Resolves an issue that prevented setting permissions on a folder alias
* Resolves an issue in which the Eject command could write to a disc in the optical drive
* Fixes an issue in which the scroll bar might disappear when deleting a file within a folder that includes files that are out of view
* Fixes an issue in the Sharing &amp; Permissions section of Get Info windows, in which the gear icon appears to be gray/disabled after authentication
* Addresses an issue in which the Show Icon Preview preference might not be not saved when turning it off
* Fixes an issue that could occur when trying to print an image from the Finder

Mail

* Addresses an issue with Message menu's Mark &gt; As Read choice
* Fixes an issue in which duplicate On My Mac folders may appear in the sidebar after upgrading to Leopard
* Improves the accuracy of the Data Detectors feature
* Resolves an issue with scrolling through a Note that is displayed using the split view in the message window
* Fixes an issue with deleting messages located in the Drafts folder
* Fixes an issue in which dragging the icon in the Safari URL field into a Mail message creates an attachment instead of a link
* Addresses an issue found when opening a item in the Notes folder that is not a Note
* Fixes an issue that may prevent RSS feeds from being delivered in Mail
* Resolves an issue in which a selected message could "flash" from blue to gray when in Organize by Thread mode
* Fixes an issue with scrolling between multiple To Dos in an email message
* Fixes an issue in which the body of email messages with certain MIME structures may not be displayed
* Improves performance with America Online (AOL) account-based messages in Mail
* Addresses issues with some ISPs during automatic set-up in Mail
* Addresses an issue in which Mail might not send mail on some networks to some SMTP servers
* Mail now automatically disables the (unsupported) third-party plugin GrowlMail version 1.1.2 or earlier to avoid issues
* Adds an option to view large icons in the Mailbox list

Networking

* Addresses a hanging issue that may occur when connecting to an AFP network volume

Parental Controls

* Improves stability when opening the Parental Controls System Preferences pane
* Fixes an issue that may prevent changes to the email address for permission requests
* Addresses an issue with printer administration for a guest account enabled with Parental Controls
* Addresses an issue with setting printer administration privileges from another Mac on the local network
* Fixes an issue that could prevent certain applications from being allowed
* Addresses accuracy issues with the web content filter

Preview

* Improves stability when scrolling through a PDF document
* Fixes an issue that prevents tabbing within a PDF document after clicking on the PDF
* Improves the Mail Document feature so that email attachments are more reliably created from Print Preview

Printing

* Addresses an issue in which remote printers may be deleted when the computer is put to sleep
* Improves printing performance when using some Microsoft Office applications
* Resolves an issue with some printing options, such as landscape orientation, number of copies, two-sided printing, and so forth that may not have functioned with some printers shared by Microsoft Windows
* Adds support for certain printers connected to the USB port of an AirPort Extreme or AirPort Express base station
* Resolves a stalling issue that could occur when installing certain Canon printing software from a disc

RAW Image

* Adds RAW image support for newer Canon, Hasselblad, Leaf, Nikon, and Sony digital SLR cameras, as detailed in this article

Safari

* Addresses issues with Safari reliably resolving certain domains

Login and Setup Assistant

* Addresses an issue in which Setup Assistant could unexpectedly appear each time Mac OS X 10.5 starts up
* Improves stability and performance during log in

System

* Improves the accuracy of the grammar checker
* The computer will now shut down if an automatic disk repair does not succeed during startup

Time Machine

* Adds a menu bar option for accessing Time Machine features (the menu extra can be enabled in Time Machine preferences)
* Improves backup reliability when computer name contains slash or non-ASCII characters
* Fixes an issue in which the backup disk displayed in the Finder may be out of sync with the disk chosen for Time Machine
* Addresses issues in which some external drives are not recognized by Time Machine
* The status menu now appears by default

Other

* Improves general stability when running third-party applications
* Addresses an issue in which the incorrect search results may be displayed for certain Automator Find/Filter actions
* Addresses an issue with the Latvian and Russian keyboard layouts
* Addresses an issue in which the backlight could turn off before Energy Saver's backlight setting

<strong><a href="http://www.appleinsider.com/articles/07/11/15/apple_releases_mac_os_x_10_5_1_update_for_leopard_users.html" target="_blank">10.5.1</a></strong>

AirPort

* Allows password-protected accounts on AirPort Disks to show up on in the Finder's Shared Sidebar.
* Resolves an issue with saved passwords for wireless networks.

Back to My Mac

* Improves the reliability of Back to My Mac-enabled Macs appearing in the Finder's Shared Sidebar.
* Improves compatibility with D-Link NAT gateways.

Disk utilities

* Restores the functionality of the progress bar during permission repairs in Disk Utility.
* Addresses an issue that could produce an alert when creating disk images using Disk Utility or Terminal.
* Improves disk partitioning when multiple RAID sets are created on the same disk.

iCal

* iCal alarms are now more reliably delivered via email.
* Resolves an issue when inviting attendees via a CalDAV account.

Mail

* Improves stability when resizing columns in the message viewer or switching between Stationery templates in email messages.
* Addresses an issue in which attachments enclosed inside an HTML link may not be clickable in email messages.
* Fixes an issue with email accounts added using the "Simple Setup" feature in which messages cannot be sent due to an SMTP connection failure.
* Improves Smart Mailboxes compatibility with .Mac Sync, and addresses an issue with To Do's disappearing when using Smart Mailboxes.
* Resolves an issue with syncing Mail accounts with .Mac in which multiple On My Mac folders appear in the Mailbox pane.

Networking

* Addresses an issue in which Microsoft Windows shared folders may be read-only when connected via SMB.

Printing

* Resolves an issue in which user-selected values on Paper Feed PDE are reset to default while saving a custom preset.

Security, Firewall

* Addresses a code signing issue; third-party applications can now run when included in the Application Firewall or when whitelisted in Parental Controls.
* In Security preferences' Firewall tab, the "Block All" option is now called "Allow Only essential services".
* Includes recent Apple security updates..

System and Finder

* Addresses a potential data loss issue when moving files across partitions in the Finder.
* Resolves an issue with login after turning off FileVault for a specific user account.
* Improves compatibility with Adobe Flash-based uploaders used by .Mac Web Gallery and certain other websites and applications.
* Resolves a potential text drawing issue with certain Adobe Flash-based websites and applications.

Time Machine

* Addresses formatting issues with certain drives used with Time Machine (specifically, single-partition MBR drives greater than 512 GB in size as well as NTFS drives of any size and partition scheme).
* Resolves an issue in which files restored in Time Machine may be restored to the backup hierarchy rather than the folders to which they belong.