BGR has just reported that Apple is testing iPhone on T-Mobile USA. They’ve published a photo of an iPhone prototype working on T-Mobile’s 3G network. Currently iPhone (assume its unlocked) works only on T-Mobile's slow EDGE network as it does not support 3G network.
The iPhone prototype working on T-Mobile’s 3G network is codenamed N94. The GSM iPhone 4 was codenamed N90 while the CDMA iPhone 4 was codenamed N92.