I looked at doing this for my HT setup...Intel Mac Mini running XBMC but i had some concerns about the onboard video setup, plus I'd have to immediately add hard drive space and some other things so I ended up just building a HTPC running Vista instead. Still, if youre looking for small and unobtrusive its not a bad way to go.
I was actually just reading a blog the other day about a guy who's running Vista on his Mac Mini as his sole home theatre setup...it was pretty damn nice.