The smart thing for Valve to do, in the course of releasing their own platform, would be to artificially handicap Steam games on Windows. Regardless of the relative merits of Windows versus the Linux distro on the Steambox, Valve could make
sure that their first-party titles never break 59.2 FPS on Windows.
It's no secret that Valve is worried about Microsoft's strategy with Windows 8, but Microsoft should be batshit terrified of Valve's strategy. They could lose ground in both the console gaming and consumer OS spaces if they aren't careful.
Whatever happens, in any contest between Steve Ballmer and Gabe Newell, I know who my money's on. If Ballmer were smart he would look to what made Microsoft successful in the first place, and choose incompetent enemies.