UPDATEx3: Apple has also posted the Mac OS X Leopard Server 10.5.3 Update [1] as well as Admin Tools 10.5.3 [2] and a Security Update labeled "2008-003" for both Client [3] and Server [4] versions.

UPDATEx2: The Google sync feature appears to work only on Macs with an iPhone configured. The iPhone sync triggers Yahoo and Google sync with Address Book.

UPDATE: First user reports indicate Apple has added Google/Address Book sync on this update, you can find the new feature in Address Book's preferences.

Apple has posted the anticipated [5] Mac OS X Leopard 10.5.3 Update fixing over 200 bugs and improving performance in several areas.

Here is the description provided by Apple (follow the links):

The 10.5.3 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

For detailed information on this update, please visit this website: http://support.apple.com/kb/HT1141 [6].
For detailed information on security updates, please visit this website: http://support.apple.com/kb/HT1222 [7].

The update weights as much as 420MB for the incremental one and 536MB for the Combo and is available either through the Software Update utility or on Apple's support web site