You can pay full price at the AT&T store, as long as you are an AT&T subscriber. Can either pick one up at the store (if you're lucky) or pre-order one and have it shipped to your home.
Asked the same question at a AT&T corporate store today. Hoping to trade in my 4 and 5S for a full-price 6.
But is the off contract tmobile version unlocked? I would assume not since it has "tmobile" tagged to the off contract term.......? Could be wrong though.
The T-mobile version, if bought through the apple store, is unlocked. It comes with a T-mobile sim inside it, but just take it out and put in your at&T sim.
If you're going to pay full price, make sure it's an unlocked phone.