The iPhone 5 aka iPhone 4S is rumored to offer support for all major carriers. In May an analyst reported that an iPhone 4S would work not only on AT&T and Verizon, but also on Sprint and T-Mobile. 9to5Mac reports that Apple is indeed in advanced testing of an iPhone for Sprint.
Apple has supposedly Sprint cell towers on campus since late 2010 for testing. These testing rumors also foregone the Verizon iPhone. An iPhone 5 that will work on all four major carriers in the US would give Apple yet again a nice increase in potential market share.
The question is if Apple is indeed able to make an iPhone that works on all four carriers or if they need two models. The latest leaked model strings in iOS 5 point to two iPhone devices (iPhone 4,1 and iPhone 4,2).