The iPad will only work on T-Mobile's US network in EDGE mode.
The chipset in the currently announced 3G models of the iPad support UMTS/HSDPA 850/1900/2100 MHz. T-Mobile's 3G network does not operate w/in these supported bands.
Some users have been confused by the fact that the iPad supports UMTS/HSDPA 2100 MHz while T-Mobile operates 3G in the 1700 MHz and 2100 MHz bands, which seems to indicate that the two would be compatible. In reality, however, the 2100 MHz band supported by the iPad is the more globally standard European/Asian UMTS band, while T-Mobile operates on more closed AWS band component of the 2100 MHz spectrum.