Actually Tmo's 3g is challenging as it runs 1700/2100 and there are very few phones with that 3g frequency. As far as I can tell, other than the phones available through Tmo, if you buy one elsewhere outright, only the sony xperia x10, nokia n900 will have the 1700 band. It's a pain as that limits what 3g phones can be used with Tmo.

ATT's 3g is more accommodating with 1900 and many phones have that band.