This pertains to OS X Address book on 10.3.3, but I would suspect that it works on others. I also believe you have to have iSync installed. The reason you would do this is because you overwrote your Address Book while trying to sync your Palm device or etc. Here's what to do.
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.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040501204700771