iSync normally restricts your list of groups to synchronize to regular ("dumb") groups. But if you create a dumb group, and drag the icon for a smart group into it, iSync will now be able to sync with the smart group.
I use this to only sync entries with a phone number to my Bluetooth phone, as an example.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060323153916446