T-Mobile USA is using a different frequency than AT&T, and from what I've read, it is also different than most overseas carriers as well. Someone can correct me if I am wrong, but a quad-band HSDPA phone won't work on T-Mobile 3G but it would on AT&T, because AT&T is using one of the traditional HSDPA bands while T-Mobile bought a different one because that was what was available at auction.
Assuming all that is true, no amount of hacking is going to get it onto T-Mobile USA, because Apple will not be covering every single radio frequency. Most likely the iPhone will have a radio that covers quad band HSDPA, which is what would work worldwide (except T-Mobile USA since they bought off-band frequencies).
If T-Mobile IS using a traditional band, then yes it will work. I'd research long and hard on what T-Mobile is using, because I thought it was a different band than everyone else.