Virgin Mobile unveiled a mobile broadband plan earlier this week that's surprising in its rarity: prepaid broadband in increments of 100MB, 250MB, 600MB, and 1GB. The plan requires a $150 Broadband2Go USB dongle, which will only be available at Best Buy. Only drivers for Windows XP and Vista were announced.

The service requires no contract commitment. A 10-day, 100 MB plan is $10; all other plans last 30 days and cost $20 for 250MB, $40 for 600MB, and $60 for 1GB. While these prices are steep relative to the amount of bandwidth offered, the lack of contract commitment may make this an affordable option for a regular, but not constant, traveler who needs ubiquitous access.

All four major US cell carriers offer bandwidth-capped monthly service plans that require a 2-year contract, and cost $60 per month for a maximum of 5GB of use. Some carriers allow overages, typically at $50/GB billed in MB increments. Early cancellation comes with some hefty fees.

Verizon Wireless offers the only moderately comparable offering, a $15 one-day pass with no specific data limits noted. Customers have to supply a Verizon network modem, such as one built into certain laptops or purchased separately at full price.

Virgin Mobile is a MVNO (mobile virtual network operator), using Sprint Nextel's network for voice and data. Sprint is a minority investor in the service. The USB modem, a Novatel Wireless Ovation MC760, includes a slot for microSD cards.