I'm still unsure with what your infatuation with Linux here is. As mentioned, Linux is a kernel that is in use for lots of totally random and different things. The userland (stuff outside the kernel) is what is interesting and the different aspect, it's the main piece of software that separates the sniper rifle from the supercomputer.
I'm not sure why you keep assuming that the userland that runs on SteamBox will be available to people without SteamBox. A unified, optimized and coherent software and hardware experience for video gaming and media is what Valve will probably trying to sell. It is too early to tell, of course. But it is a bit different than building a Windows machine and slapping some blinky lights to it and selling it at a premium as a "gaming PC". Dell and HP can do that just fine.