I'm sure Apple worked out something so that there is an obvious difference in the connection so that the carrier can bill accordingly.
So for those using PDANET, AT&T probably can't tell.
But for those enabling the new Apple supported tethering, I'd be very afraid. All it would take is for the OS to change MAC addresses base on the connection type (standard data vs tethering.)
It's not hard guys....to assume they can't tell is not only naive, it's just plain foolish.