Courtesy of Apple
Reliability improvements targeted Apple’s sync capabilities in MobileMe, as well as Address Book, portable home directories, and other items that use Apple’s sync services. Performance improvements targeted graphics performance specific to iChat, Aperture, iTunes, and Cover Flow. The graphics update is also said to correct problems some users have encountered with ATI video cards.
OS X 10.5.6 also includes a number of security updates. Most of the issues addressed revolve around the potential for maliciously crafted files or web pages to cause denial of service attacks, random crashes, or possible code execution.
OS X 10.5.6 is available via Apple’s Software Update.