As far as I can see, I have never heard anyone that it cannot be used, so why not.
In fact.. it would be almost Ideal,
Got everything what you need.
Only need to figure out what kind of power it uses.
Do a search in www.mp3car.com/searchexamples.asp
I think there was someone who used an Apple powerbook in his setup, but there might be more examples...
I think when you want to take the laptop appart and use the screen a bit further away than the standard 'ribbon' cable supports you could run in some probles how to increase the cable length and what sort of cable to use..
I think, when I had an old laptop with on-board-everything and a decent screen. I didn't have to think for a long time to decide what i would use, a standard pc or a laptop. I think it would be a laptop.. but then there's not half the fun of building it into your car.
Hope this helps you out