I'll give this a shot, but I'm not optimistic. I have an Nvidia Ion graphics card in my HTPC, and h.264 acceleration doesn't work with anything other than MPC-HC (I tried XBMC 10). It's really too bad, because the Ion chipsets are wonderful for HTPCs, and XBMC has a great UI. A combination of the two has the potential to be a great HTPC that actually plays everything (instead of the half-assedness of Apple TV or Roku).
Optimistic? You're in for a surprise. It works perfectly on my Asrock ION 330 with the Ion graphics card. Can play 1080p flawlessly without hardly affecting the CPU thanks for the VDPAU acceleration http://wiki.xbmc.org/index.php?title=HOW-TO:Install_XBMC_on_...
My HTPC had an Ion, and I had flawless hardware decoding of 1080p H.264 in XBMC on Linux about 2 years ago. Unfortunately, I can't give you an easy step-by-step guide, because I spent like a week tweaking the system, but it was basically a then-recent build of XBMC with hardware acceleration provided by VDPAU. It was an amazing setup. (Everything is in the past tense because I've since moved to San Francisco and don't have room for a TV.)
I've had XBMC 10.1 running on an Acer Aspire Revo R3601 (which I think is Ion based) for a couple of years now and it's been able to smoothly play everything I've ever thrown at it. The trick is to get VDPAU working.
My HTPC has been out of commission for over a year, and I used to run XBMC 9.x on on a Nvidia Ion board with full GPU acceleration. (At least I'm pretty sure the Atom chip wasn't decoding 1080p all by itself...)
