The $27/month via AT&T Next doesn't lock you into a 2-year service contract.
Actually, the current AT&T Next offering isn't bad, they simply take the full price of the phone and divide by 20 or 24 months.
If you cancel service before the end of the period, you owe the balance of the price of the phone. It's essentially a 0% loan for the phone.
(BTW, the $27/month installment price means the Fire Phone's real price is ~$650)