Massachusetts based WAAV offers cellular routers that give you Wifi connectivity in your car.

With Wifi in your car you could use the new iPod touch and buy iTunes songs while on the way to school or work.

The lower cost AirBox CM3 has a connection to a cellular provider on the back-end, and it provides a local Wi-Fi hotspot for clients. Users can also connect directly to the unit with Ethernet.
The more expensive AirBox X2 has two connections to cellular providers on the back-end, and it can also provide a local wired or Wi-Fi hotspot. This is ideal for applications requiring greater redundancy, coverage area, bandwidth, or number of users.

Based in Cambridge, MA, WAAV has been the leader of innovative products for in-vehicle Internet. Formerly Omniwav Mobile, they introduced the first Wi-Fi hotspot for a vehicle in Jan. 2005. This year at CES, they released the world first multiple connection cellular router, the AirBox X2. The X2 allows many users at once, such as a bus full of passengers, to surf the Internet at the same time.

The WAAV AirBox CM3 retails for $499 and the AirBox X2 retails for $1099. You need a dataplan for the Airbox. WAAV charges $59.99/month for unlimited use.
Via the WAAV site.