[GadgetKen]

Prepaid 3g data service in the US is not as widely available as in other countries...whether it's GSM or CDMA based. Tends to be more postpaid long-term contract or recurring monthly postpaid. Prepaid wireless broadband data (the heavy duty type used on laptops; not just handset web surfing) is relatively new here.

Suggest the following for prepaid nationwide coverage:

VirginMobileUSA MVNO using Sprint CDMA network: Mobile Broadband Service | Virgin Mobile Could be activated and left dormant for up to one year. Caveat is data is best for light usage (rates are a bit expensive) and is limited to the native Sprint network without off-network roaming (but the good news is it is largely 3g).

AT&T offers a prepaid broadband plan but 3g coverage is spotty and they are a bit expensive: AT&T offers prepaid wireless broadband | Signal Strength - CNET News

Verizon CDMA offers a prepaid broadband plan with extensive 3g coverage but it's pricey: Verizon offers prepaid wireless service for laptop users | Signal Strength - CNET News

T-Mobile offers 3g (or in some cases high speed 4g) through resellers like Datajack and others, but coverage areas are somewhat limited.

I use an ISP in Oregon called Millenicom: Welcome They do recurring credit card billed 3g data service on Sprint, T-Mobile, and Verizon. I use their 3g Verizon service (US$60/month for 10 gb of data). It's a good service but it's not true prepaid.