Apple decided to leave out hardware support for the radio band that T-Mobile will be using for their 3G system. So while you can use it on T-Mobile's voice and EDGE network, 3G on T-Mobile won't work, and since it's a hardware limitation, can't be hacked to make it work either.
I'm pretty sure these tactics are illegal under United States antitrust law and have been for over a hundred years.
Either way, though, I know for a fact that they don't deserve my business.
