There is quite a few people doing it now in England, but not as many as the US, I had to import my carpc from US.
It's so much better than a tomtom

World mapping for a start.
Once you have a PC fitted under the seats all you can see is the Screen, apart from a USB hub in the glove box and a mini bluetooth keyboard.
I have Night vision Cams front and rear, 500gig hard drive for music vids etc, internet, TV, bluetooth for mobile link (comes out speakers and mutes music) MSN mesenger with webcam wjile driving lol, you just keep adding bits here and there

Non of which costs much of the net either.
Just think what someone like BMW would charge for some thing half as good on a new car?