Mac OS X 10.5.3 now lets iPhone users sync their Address Book with Google Contacts, according to the Google Mac Blog
We're happy to tell you that starting today, 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. Itll ask for your Google account and password, then automatically update your contacts every time you sync your iPhone.