Tethering places demands on AT&T's already strained wireless network. It's also possibly why AT&T's Unlimited MEdiaNet PAYG data plan is due to be discontinued.
NetShare, a tethering solution was removed from App Store for that reason. Tethering is not supported on any current AT&T iPhone plan. Some AT&T contract data plans include tethering support for a monthly premium, usually about $15 above other costs that's different from dedicated EVDO type solutions, which are meant to provide on-the-go data for laptops.
