Reports on TechCrunch say that the impending iPhone 5 will run on both GSM and CDMA networks. CDMA is the standard for Verizon and Sprint, GSM runs AT&T and T-Mobile (as well as the eventual combined company). So right now there are two completely different iPhone 4s being made, it's just that they look completely the same to any user. This World Phone feature has been rumored for a long time but TechCrunch has apparently spoken to developers who passed along something about the specs under which they are building.
If it's true, it would seem to broaden the possibilities of the iPhone to more easily accommodate Sprint and T-Mobile. Also, if you wanted to switch your coverage, you wouldn't need a new device. It could also make it a lot easier for Americans to travel abroad and get coverage.