The image obtained by BGR is, of course, just a prototype, but it's a prototype running on T-Mobile's 3G bands. It's also white, and according to 9 to 5 Mac, might even be one of the iPhone 4S developer prototypes we wrote about earlier.
Those prototypes are being given out to game developers (perhaps others, as well) so they can test their software on an iPhone with an A5 processor (the same as in the iPad 2).
While we have little doubt the prototype that BGR has an image of is legit, it would make little sense for a T-Mobile iPhone to be produced. After all, AT&T has already announced that current T-Mobile customers will have to get new devices (eventually), as the frequencies used for T-Mobile 3G will be used by the combined company for 4G.
Of course, that will take some time, so perhaps it's not such a bad idea after all (a T-Mobile 3G-enabled iPhone).