Beware that tmobile and AT&T use different 3G frequencies. You will be limited to EDGE usage on tmobile.
I just picked up a cricket 3G USB modem and will be soon ordering a Cradlepoint PHS300 to leave in my car. Since anytime that I want to use my wifi iPad will be on the road anyways, it is a good setup for me.
I would have bought the 3G iPad but, I didn't want to wait and I didn't think it would work on 3G in the countries I'll be spending most of my time in over the next few years. This is a decent compromise for me(eventually to be replaced with a Colombian iPhone, tether to that when needed)