Android zoomed past Apple in countries where iPhone is available on all the major carriers. It's not only carrier exclusivity harming Apple -- it's all the restrictions and the fact that the iPhone just isn't a modern smartphone.
It lacks open development, Flash, Java, real HTML5 and other internet standards that the competition has. The FaceTime stuff is a clone of video chat from 5 years ago, and is implemented on a whole range of phones (including Sprint's excellent 4G handsets). Apple is hoping that releasing its handset on the #1 US carrier will keep them relevant and save them from slipping underneath the waves.
However, it also poses a downside. Lots of users will realize that a great deal of the iPhone's problems -- poor signal retention, dropped calls, slow data speed, crashes and poor performance -- are a result of Apple's bad engineering and not AT&T's network as has been argued in the past.