If you're using Mac OS X 10.4 Tiger, please note that Smart Groups that you create in Address Book are not syncable, even though many other types of contact information are.
Because of this, you may notice that when you select a device in iSync, you can't choose any Smart Groups from the Contacts-Synchronize pop-up menu.
As a workaround, create a regular group (not a Smart Group) of contacts in Address Book that's just for iSync syncing. Add the contacts that you want synced to this group. Then choose that group as the one to sync in iSync.
Note: Any contacts you create on your phone or other devices will still show up in relevant Address Book Smart Groups if that contact meets the Smart Group's criteria.