Not quite. There have been 3 different data cards out that aren't t-mobile branded, but that DO work with T-mobile. Last summer there were 2 devices that were mentioned on HowardForums, that were meant to work with a little wireless ISP in the midwest somewhere.. that happened to be T-Mobile 3G compatible (one of them was a bandrich card, one was a specialty Option card).

The third one went through FCC approval recently, and has specific support for T-Mobile 3G.

So, it's not that it has to be from T-Mobile, it just has to specifically be for T-Mobile's 3G AWS spectrum. The mis-named "North American 3G" bands aren't what T-Mobile uses for 3G.