Sigma Designs 8634 based head unit
It's not a Car 'PC' per say (e.g. x86), but it will run Linux and Windows CE. Just started planning a build using the smp8634 based multimedia player from Syabas' Popcorn Hour B-110 main board. Has just about every hardware feature I could ask for all fitting in an in-dash only space with minimal power (~20W) and heat yet still being able to play any video codec at any resolution you throw at it, super low boot times (~8-10s cold), and a full OS (Linux in my case). Although I wont be running the firmware that comes on the board (NMT) in favor of my own. The SoC integrated MIPS 4kec core @ 300 MHz can be a tad slow for transcoding recorded radio and ripped cds, but should be adequate for general UI, nav, handsfree, media player/browser, and whatever else I integrate.
SDK is still under NDA but can be found if you stare long enough at .cn domain names. And the intellectual iron curtain is opening up slowly on its own. Kernel, rootfs, toolchain, directfb, and other infrastructure software legally available. Can read more about the start of the build here (preliminary pictures included).