You can also try Plex, which, like Boxee, is based on XBMC but with yet a different UI.

While I use a Mac as my primary machine, and would only ever continue to buy them in the future, I don't think I could see myself dedicating one to duty as a media system connected to the TV.

It's super difficult to support multiple platforms out of the gate, but hopefully they've created it in such a way as to allow for porting later on. If they want OEM contracts they'd really have to.

I've tried Boxee on my Mac, but honestly, the only reason I keep or use any similar programs on the Mac anymore is strictly for testing purposes. MPlayer is what I use to quickly play back a single video on the desktop when needed.