I have my Address Book configured to sync with Google Contacts, and I sync my iOS devices with my Mac.
There is also a "Sync Google Contacts" checkbox in iTunes, in the Info tab for the device. After selecting it, it asks for a Google account/password.
What exactly does this checkbox do? When I left it unchecked, my Google contacts from Address Book are synchronized anyway. When I check it, I cannot see any difference. It's confusing.