I too am interested in doing a car stereo with a Pi/touchscreen. I'm no programmer, but I am a sys admin, and know my way around the Linux OS pretty well.
In my case, I have plans to use my stereos existing wiring harness for power. Problem is, my stereo turns off as soon as the key is turned off, and I don't want to subject the pi to that multiple times per day; it needs a graceful shutdown. Does anyone have any wise ideas about this? I found another thread that dealt with gracefully shutting down a pi thermostat in the event of a power outage by using several neat little circuits running on either batteries or super caps that tells the pi to shutdown when it detects power loss, but they didn't recommend using their circuit in an automotive application.