The problem with getting locked up to a new two year contract now is that Nokia is rumored to be coming out with the Windows Phone 8 flavored Nokia Lumia 1000 EOS model for AT&T with a 41MP PureView camera. On the other hand, if you don't mind the handcuffs, and really want the Nokia Lumia 920 for $19.99, it might worth leaving your current carrier for the deal. That needs to be your decision. By the way, the price includes free two-day shipping.
source: Amazon via TheNokiaBlog