Ordered mine from RS. I have seen plans on the web somewhere for a DIY box. Basically you print it on some cardboard and cut it out and fold it into shape. Has all the cutouts for USB etc. actually looks better than it sounds.
Mine came with a USB mains adaptor and an sd card with Linux installed on it.
I have a radioguy dd unit, which I was just going to plug the video out of the Pi into the av in on the dd unit. However, I have just bought a rear roof mount DVD screen and headphones. It has 2 video in plugs, so I am going to connect one of these to the Rasberry Pi, and one to the dd unit. The Pi will probably live in the glovebox with a USB Bluetooth adaptor plugged into it. A standard 12v to 5v USB adaptor will power the Pi. I will use a Bluetooth keyboard/mouse or similar to operate it. I'll install a media program (xbmc or similar) onto it and have films etc loaded onto a USB stick or drive.
That's my plans
Hopefully it'll work