Mac OS X 10.5.3 makes dozens of improvements and bug fixes. Most notably however, is a feature that was not on the release list. The feature adds Google contact synchronization to Apple’s SyncServices, which also appear inside of iTunes. This means that you can now synchronize your contacts with Google, just as with Yahoo previously. Yahoo synchronization was added alongside the release of the iPhone.
Apple encourages all users to update to Mac OS X 10.5.3.
Alongside 10.5.3, Apple released their sixth beta of the iPhone SDK. The 1.19 GB release “fixes many bugs and add [sic] support for the latest iPhone OS.” Concurrently, iPhone SDK Beta 6 requires Mac OS X 10.5.3 to be installed.