Mobile Share Plan only
It is all but confirmed that this is a feature of iOS 6 for iPads on the AT&T Mobile Share Plan. AT&T advertise that the hotspot function will be enabled on all compatible devices in a Mobile Share Plan - I guess iOS 6 is required to enable it on AT&T iPads.
I just wish AT&T had advertised that decision back when the new iPad LTE came out, and everybody was up in arms because Verizon enabled the hotspot feature and AT&T didn't. AT&T could have easily said that the feature would be enabled later in the year.
Just one more 'nudge' to get people onto their mobile share plan, which is way overpriced for individual users.