AT&T upped their iPhone tethering plan from the 4GB to 5GB, meaning an extra $10 bucks a month from the old plan but an extra $20 to go from the current 3GB plan to tethering 5GB plan. I don't expect them to play nice on iPad tethering.
I've used iPhones since September 2007 and never give a cent to AT&T, I don't plan to anytime soon.