Gene Munster, the Piper Jaffrey senior research analyst who usually hits the nail on the head when it comes to Apple forecasts, thinks that Apple will move to multiple U.S. carriers within the next year. He believe that this will happen next summer, which has been the time when Apple has introduced new models of the iPhone.
Munster noted that having multiple carriers in a market has helped Apple to achieve greater success in terms of market penetration. He pointed to France as an example; originally, Apple inked an exclusive arrangement with Orange. When it moved to a multi-carrier deal, Apple's market share in France jumped to about the 40 percent range. In the U.S., the iPhone's market share is only in the mid-teens.
Many industry pundits expect the next iPhone carrier to be Verizon, since they are beginning a transition to a new, iPhone-compatible 4G network (LTE) in the next year. This would make the transition rather simple for Apple, since they wouldn't have to design an EV-DO iPhone, but instead just use the current hardware design.
Update: Per TUAW reader Ben C., the Verizon 4G transition is not expected to be completed until 2013. In addition, a CDMA-based iPhone would need to be deployed for Verizon. Thanks for the correction and explanation.