Hi,
I also did a similar setup with coreboot on my Asrock E350M1 (the one I bought from Peter at Linuxtag) but didn't document it yet either. I am glad that someone else is trying to do the same, hopefully we can exchange tips and tricks for such a setup.
I used a plain Ubuntu, installed XBMC-git packages from a PPA(the stable version still lacks VAAPI support), then somehow got the VAAPI video acceleration work with the ATi proprietary drivers. Most of the stuff seems to work, but I have some stability issues and kernel panics sometimes when initiating video playback. I blame the XBMC version that I currently use, but the 3.0 Linux kernel which I ended up using and the ATi drivers which might not be mature either.
I use a full-blown distribution as of yet, so the boot speed performance is much worse than Marshall's, but I needed a full system that I can sometimes use as a home computer.
I will try to document this as soon as I can.
Cristi