I have often been told that these kinds of things are illegal if you can see it from teh drivers seat, but I have yet to have anybody quote me section of code that prohibits them.
Computers in the car, being used while driving is a bad thing, they will distract you. Myself I have at different times had anything from a nine inch screen built into a console to the good old laptop on the passenger seat (that is the best solution really).
Like Bigscrun mentioned a few years ago lots of the more geeky types would set these up so they could play MP3s in thier car, and maybe do some surfing in gridlock. With Ipods coming into widespread use and the major car audio manufactorers integrating them into thier systems thier just isn't as much need for a PC in the car.
The one thing I find a PC to be extremely usefull for in the car is GPS. I would rather have a 15" screen and learn the hotkeys than the dinky little displays on the standalone units anyday. The larger screen is easier to see, and you can fit a lot more info on the screen.