iPhone: carrier question

Hi there,

Little interrogation about what I have read here and there.

Should someone have a legit unlocked iPhone, which can be bought in many countries, but not in the USA, and move to the States.

Official carrier is AT&T. And I read that if I go to t-mobile, I could only get edge, not the 3G.

However, it seems to me that the iPhone is penta-band UMTS/HSDPA (800, 850, 900, 1900, 2100*MHz). T-Mob operate their 3G network on 1700 and 2100 MHz.

You saw it coming: why can't the iPhone run on t-mob's 2100 MHz 3G network? I am sure I am missing something, and would be grateful if someone could explain it to me.

Thanks!