I did a rudimentary Car PC in one of my earlier cars - using kit from LinITX.
Among other things I had a WLAN card acting as HostAP (i.e. a wireless hotspot) and then a PCMCIA 3G card for bridged internet access.
End result was that people could get into my car, fire up their laptops and browse the web wirelessly like they were in Starbucks. My car was a mobile hotspot.
Obviously, they needed my WPA key to get onto my wireless LAN, but it worked a treat!!
The plan was always movies and music, but I changed the car/priorities before I ever got that far.
I'll watch this thread with interest.
PS --> Also messed about with Centrafuse - nice bit of kit and decent API for extended development. I got halfway through a synchro application that would detect when the car was near my house (i.e. my home WLAN) and auto sync the music/video library on the Car PC HDD with that on my home server. The idea being that I could rip songs/movies to my home media server and they would auto sync with my car. Never got round to finishing it off..........might have to dig that code out again at some point. If I can find it. Those were my late night, pizza eating, mystic code writing days.....