Hmm....sounds like maybe they know people are doing just what you did...and are double checking to see if two accounts are using the same phone and resetting accounts recently activated with iPhones.
I'd say people who were legitimately upgrade eligible should contact AT&T if their status changed.
People who are trying to trick the system could probably try again...or just be glad they didn't get their accounts cancelled and pay the full price like the rest of us.
Seems like there'd be no way they can figure that out without somehow messing up accounts of those that originally bought an iphone and just legitimately sold it to prepare to buy the 3G version. Or taking away the eligbility of someone that bought the iPhone off ebay and activated it but the seller hadn't removed it from their account because they plan to upgrade to 3G too.