It would seem odd for them to launch the Linux client with the fanfare that they did and then pull the rug out from under everyone once the Steambox is launched, though I suppose it could happen. Besides making Steambox fundamentally incompatible would actually be more work than using standard parts.

Regards distros, they don't seem to mind people repackaging for other distros though it would be unrealistic to expect support for running the games under every conceivable distro of which there are thousands.