Google has implemented a function that allows the Address Book application in Mac OS X 10.5.3 to sync the iPhone Address Book with Google Contacts. A blog post from Google reads:
"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. Before you try syncing, it?s a good idea to back up both your Address Book contacts and Gmail contacts. For instructions on how to do that, plus other important information, see the FAQ."