Also, mp3car.com has a couple of power supplies that will give you the ATX 20-Pin power as well as the 4-Pin DC power.
Mp3Car DSATX 220 Watt Automotive Power Supply
M4-ATX 250W Intelligent DC-DC PSU
I'm looking at the M4, just because it's smaller. Now I'm in the process of searching for a case that will house both the Mini-ITX board and the PS (right now I have an external PS and I'd love to get it all in the case, if that's possible).