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.
Update
Folllowing issues after update (updated 8:55 wed 19th dec)
Nearly all you need to know is available here:
http://www.geisterstunde.org/drupal/?q=osx_routing
All I found is that $BUNDLE refers to /System/Library/SystemConfiguration/Kicker.bundle/ and the script should go in $BUNDLE/Contents/Resources - not $BUNDLE/Resources (the paths are a little mixed in the original article).