tho a Rasberry Pi small form factor PC will load up XBMC half the size of any of these (shame the world wide demand has outstripped thier supply for awhile) but once they get production in full swing + cases they will be selling like hot cakes.
http://www.raspberrypi.org/
for me my HTPC is about lookinfg good and working good...
As it happens, I also have the Fusion running my HTPC, running Windows 7. But power consumption isn't that low. I'm considering a Popcorn hour C300 or a Dune with Blu Ray support, ideally using the new Sigma chipset.
My main computer will likely be doing the recording, and stream to PH or Dune in the bedroom, and the PS3 in the media area (the QNAP runs Twonky or PS3Media Server)