This nice, short article from Wired reviews AutoNet Mobile. They have a wireless router that talk on both the Sprint and Verizon networks, plugs into your car’s cigarette lighter or into an AC adapter (in your hotel room, for instance) and gives you relatively high-speed Internet access (600kbps-800kbps, typically). WEP encryption but it’s better than nothing. No pricing on the site but you can get it for about $11/day when you rent a car from Avis in their Avis Connect program.