Apple’s release of OS X Leopard 10.5.3 Update last week now allows users to sync their Mac OS X Address Book with Google Contacts.
“We’re happy to tell you that starting today, [May 28] it’s easier to sync up your contact lists. The Address Book application in Mac OS X 10.5.3 now lets iPhone users sync their Address Book with Google Contacts. To try it, go to the Address Book menu, choose Preferences, and then check Synchronize with Google. It�ll ask for your Google account and password, then automatically update your contacts every time you sync your iPhone,” Mike Morton wrote for the Google Mac Team. 
The feature is only available in OS X Leopard, version 10.5.3 or later.