How to sync your Mac's address book with Google contacts
Make sure to backup your Apple Address book and export all Gmail contacts first. Then change the Family ID value to 10001 (at least for the file located in a subfolder of your home directory), launch Address book again, check “Synchronize with Google” and fire up iSync to complete the process. By the way, if you’re on 10.6 already, you do not need to get your hands dirty. It’s enabled by default.
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Source: Lifehacker
