DataJack looks interesting - "unlimited 3G" for the USA for $40 a month, prepaid with no term commitment (plus $100 for a 3G quad-band 850/1900/2100/AWS dongle, and $10 shipping). There does not appear to be any data cap. It has fallback to GPRS/Edge outside 3G areas.
Their coverage map
DataJack Coverage is essentially useless (since it can't be zoomed, and entering my US zip code just gives "call us").
But the coverage map matches that at
T-Mobile (data). So the (tiny!) yellow dots are T-mobile 3G, the brown area is T-Mobile GPRS/Edge, and the pale purple matches T-Mobile roaming (so is also just Edge). So it seems to be just on T-mobile and their roaming partners (so no 3G coverage where I go, yet).
Their "retail store" coverage is somewhat sparse at the moment.
With the option to switch access on/off (in full-month chunks) it could be very useful for part-time residents of the the US, and visitors, if you're in one of the few 3G areas. That's if it works, and doesn't later have a hidden fair-access-policy for 5GB a month or less! There's no limit stated yet.
They claim they are planning international roaming later.
Rob