Tranquil PC (via mini-itx.com) have this cool looking Media Center PC available in the UK. It's very small and so has some limitations but the design of it is great. It's using a VIA mini-itx platform so it's not capable of playing HD WMV and only supports one tuner, but for a basic Media Center PC it looks pretty good. The specs are:
All in all a pretty impressive little system and right now mini-itx.com are running a competition to win one (I'm not sure, but it's probably UK only).