Playing with Address Book on Mac and the syncing feature, I have found a bug/memory leak into the AddressBookSync process.
I’m using Snow Leopard 10.6.4 and after some sync with my phone/OSX/etc, I have found my Mac completely unusable, with the CPU and RAM saturated by the AddressBookSync process.
To fix this annoying behaviour you have to play with Safari. Yes, this bug on AddressBook Sync is due to a Safari recent update.
Open the preferences panel in Safari, go to the Autofill tab and remove any check box. Restart your Mac and the bug should be gone. 🙂