"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."
Looks like the ability only exists for people with iPhones. I don't have an option to sync with Google and I don't have an iPhone. At least I learned that you can sync with Yahoo/Exchange (Address Book -> Preferences).