The No. 4 U.S. carrier has begun selling micro SIMs in its online store. With a Web-only discount, the micro SIM is free.
However, folks considering paying the premium, and it's steep, for an unlocked iPhone should realized that on T-Mobile, the device will run on EDGE only. That's 2.5G, and therefore SLOW (really slow).
That said, there's been talk of both a Sprint and T-Mobile version of the iPhone, coming perhaps when the iPhone refresh comes in the rumored September / October timeframe. If a T-Mobile iPhone were released then, it would definitely support the proper 3G frequencies for that network.
That said, it's all still just a rumor, one of many that floods the Web just prior to an iPhone launch.