iSync's sync data must be messed up, I thought. I wanted to trash the sync data and let iSync start anew, but I couldn't find it. Turns out it was stored within the main Library folder, here: /Library -> Application Support -> SyncService -> nnn -> EngineData. The "nnn" is the uid -- an integer number used internally -- of your account. If you don't know the uid of your account, you can find it using NetInfo Manager (in Applications -> Utilities).
I trashed everything within the nnn folder (with iSync off, of course), restarted iSync, and voila! iSync started jiving with Address Book correctly again.
Now, somebody with a little bit more iSync-savvy can explain what within the EngineData folder was messed up; and how trashing the whole nnn folder was overkill. But I'm just happy I got the contacts back in Address Book.

