The latest update to Mac OSX 10.5.3 now includes inbuilt google/address book synchronization.
However - after updating and adding my login details for gmail to addressbook it just seemed to hang with:
Can't connect to the sync server: NSInvalidReceivePortException: connection went invalid while waiting for a reply ((null))
The solution was to remove the contents of ~/Library/Application Support/SyncServices/Local and try again.
Note that for account name it needed the full e-mail address complete with googlemail.com domain.
Each and every sync is causing large numbers of duplicate entries - I have 299 cards right now - each sync it goes up to 362 cards. Extremely annoying.