It was huge news when Apple announced that the iPhone would be available on a second carrier in the US. The deal with Verizon so far has been fruitful: around 2.2 million iPhone’s have been sold on big red already. Without an exclusively agreement in place (the one with AT&T expired early in 2011), Apple is free to explore partnerships with other carriers. While doing so might add complexity to Apple’s ability to roll out new hardware, since they must make a slightly different version for each carrier, having the iPhone on more carriers means more sales.
And it looks like T-Mobile USA might indeed be the next carrier to get the iPhone. BGR has come across a protoype iPhone 4, designated as the N94 variant (the original GSM iPhone was N90, and the Verizon version is N92), which works on T-Mobile. Since we’re close to the end of the lifespan of the iPhone 4, it’s very unlikely that we’ll see a T-Mobile version of the phone, but rather, Apple has its sights set on doing a T-Mobile version of the iPhone 5 (in addition to AT&T and Verizon).