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.