Change to your user's Library -> Application Support -> SyncService -> LastSync Data -> Address Book folder. In the Terminal, that would be cd Library/Application Support/SyncService/LastSync Data/AddressBook/. Then copy the AddressBook.data.previous file to your user's Library -> Application Support -> Address Book folder, and then rename it AddressBook.data. Again, using the Terminal this would be:
cp AddressBook.data.previous ../../../AddressBook/AddressBook.dataRestart Address Book and you should be ok. It appears that (or at least diff thinks so) AddressBook.data and AddressBook.previous were identical on my machine in the LastSync folder and either could be used (so long as you rename AddressBook.data.previous to AddressBook.data). Your results may vary.
This also may assist only to the point of you restoring your Address Book up the point at which you last synched your device. For those of us with iPods or Palm devices, this is probably fairly often.

