I built a rig inspired by a coworker for my EeePC in my car. It works a lot better than I imagined it would, and doesn't look that bad. It's a bit ghetto at the moment, but the foam tape is temporary (ACE didn't have the foam tubing I was looking for).
t0ast and I loaded drove until we got hungry, and about 800 networks later.
I would like to find out if the digital billboards have wifi, you can imagine where that's going ; )
The gear: My wifi bag, my Eee and a USB GPS receiver.
Car Rig: Made from steel pipes, and foam tape. Bolted to the floor.