I am going to start my carputer soon as I am recieving a free old laptop from a friend tommorow and had an idea. Instead of using a classic touchscreen, why not use your iphone / ipod touch which accesses your computer via remote desktop and then controls everything from there.
Remote desktop is possible on the iphone, see:
(Edit: dangit the site automatically took out the time, well its at 5 minutes and 20 seconds or so)
So I am going to try this out, tell me what you think of the idea, if you believe its possible or not, worth it, or has already been done!
Edit: Buying my power inverter and getting the laptop tommorow! Let's see how this goes, I will post updates in the project section)