I thought AT&T allowed tethering, but for a fee. I could be wrong though since I'm with a different carrier, but I remember it being for a fee when I was looking at them a while ago.
AT&T has tethering on the iPhone. However, the latest I can find is that AT&T hasn't announced support for the 3rd gen iPad. http://www.cultofmac.com/151586/att-wont-let-you-tether-your-new-ipad-lte/ I'm hoping AT&T will release something about tethering this week, but they're not known for their rational or generous data policies.