Wow. There are a lot of people on this board that DON'T understand how cell phones work.
A UMTS iPhone using T-mobile's frequencies would be easier to make than a CDMA Rev.X iPhone because the current AT&T iphone is already built with a UMTS chip.
So, instead of loading the manufacturing robot with 850, 1900, 2100 MHz chips, they would load it with 850, 1700, 2100 frequencies.
Done. No retooling the radio interface or replacing innards as a CDMA Rev.X phone requires (remember, they don't use SIMs, they read the EIN of every phone).
The reason why Apple hasn't tested a T-mobile iphone is because it is exactly the same hardware as the AT&T iphone.