T-Mobile uses different 3G bands, so the iPaq 910 won't work on their 3G bands (it will work on GSM and EDGE). T-Mobile uses the 1700/2100MHz bands and AT&T uses 850/1900MHz. T-Mo just started their rollout of 3G this year in the US, so there aren't many 3G phones for their network (pretty much just what you see listed on their web site, and no cool imports). The G1 is the closest thing they have to a 3G smartphone at the moment.
-------------------- Lisa Gade Editor in Chief, MobileTechReview