This is going to be awesome. My XBMC media center PC is just a core OS with minimal X. I'll add a controller to it, and a launcher for Steam.
That brings me to something I've been thinking about - Xinput devices like the 360 controller and some Logitechs/etc are pretty much the de-facto gaming controllers on windows. On Linux we have the amazin xboxdrv, and the fairly crappy xpad driver. I wonder which one Steam will end up supporting? I hope to Zeus it's xboxdrv - it IS in the Ubuntu repos now, and should be default instead of xpad. Not sure why it isn't.