Mac OS X 10.6.3 does include reliability and/or compatibility issues for a number of subsystems, including QuickTime X, OpenGL, AirPort, printing, Bonjour, DNS, photo screen savers, and USB input.
It also provides fixes for a variety of specific issues affecting Mail, Aperture, iChat, iCal, MobileMe, Time Machine, Parental Controls, Directory Services, Mobile Accounts, copying to Windows file servers, Xsan, and glowing, stuck, or dark pixels in video from the iMac (Late 2009) built-in iSight camera.
One item on the 10.6.3 list that will be especially welcomed by users of some older software is that the issue that prevented files with the # or & characters in their names from opening in Rosetta applications has now been fixed.
Daylight Saving Time rules have been updated to reflect recent changes in areas including Argentina, Fiji, Pakistan, and Antarctica, and improvements have been made to the performance of Logic Pro 9 and Main Stage 2 in 64-bit mode.
A full list on non-security changes can be found here.
Please read on for the security changes delivered in 10.6.3 and in Security Update 2010-002 for Leopard.
In parallel with Mac OS X 10.6.3, Security Update 2010-002 for Mac OS X 10.5.8 provides fixes for vulnerabilities in the spell checker, application firewall, AFP server, Apache, ClamAV, CoreTypes, CUPS, curl (two issues), Disk Images (two issues), Cyrus IMAP (server only), Cyrus SASL, Desktop Services (two issues), Directory Services, Event Monitor, FreeRADIUS (server only), FTP Server (server only), iChat Server (server only; four issues), ImageIO (four issues), Image RAW (two issues), Libsystem, Mail (two issues), Mailman (updated to version 2.1.13), OS Services, Password Services (server only), perl, PHP (updated to version 5.2.12, even though the PHP project team is encouraging all users of version 5.2.x to upgrade to 5.2.13), PS Normalizer, Ruby on Rails (updated to version 2.3.5), Server Admin (two issues), SMB, Tomcat (server only; updated to version 6.0.24; version 6.0.25 - with additional security fixes - was released earlier this month), unzip, vim (updated to version 7.2.102 - 7.2.411 is current), Wiki Server (two issues), X11 (two issues), and xar.
In both cases, a significant proportion of the vulnerabilities involve susceptibility to malformed files such as TIFF images or H.264 movies.
Both updates can most easily be applied by running the Software Update utility. Alternatively, they can be downloaded via Apple's Support Downloads page.
Some users will prefer to download the combo version of the Mac OS X 10.6.3 update, as this can be used on any 10.6.x installation. While combo updaters are sometime substantially larger than the corresponding regular (incremental) updaters, the 10.6.3 combo is only 10% larger. This indicates how wide-ranging the changes are in 10.6.3.