Using iTunes 7.7 or greater, users can sync their Google contacts to their iPhone or iPod touch address book using these simple steps. Before syncing, users should organize their contact lists on their Mac, iPhone or iPod touch, and in Google to ensure a smoother transfer. Once you are organized (including having at least iTunes 7.7 installed on your Mac), connect your iPhone or iPod touch and...
In iTunes, select your iPhone or iPod touch in the device list.
Click the Info Tab.
Select "Sync Address Book contacts".
Check the box to "Sync Google Address Book contacts".
Click the "Configure..." button.
Enter your Google ID and password, click OK, and sync your iPhone or iPod touch.
You will notice that a similar process is performed for users that wish to sync their Yahoo! Address Book contacts. Before syncing you may want to make a backup of your contact lists. Use the export function from Address Book, Google, or Yahoo! to export your contacts as a vCard. For more information about syncing your Google Address Book contacts to your iPhone or iPod touch on your Mac or information for how to sync them to your Windows PC, read this knowledge base article from Apple.