Doesn't T-mobile have an all data plan for $30/month? If you bring the GSM phone they can hook you up. You could always go to an Apple store secretly slip a micro sim into an Ipad 3G and register for ATT's Ipad plan. But I think it's now capped at 2GB. Before ATT dropped the unlimited Ipad plan for $30 I managed to sign up for it. I use that sim in my 3GS and it is now Ipod touch on steriods.
I don't think anyone else in the States is offering data only. Rumors say that Verizon will be getting an Ipad type device so maybe they will roll out a data plan too. Being CDMA changing devices will be a pain in the butt.