This is nice in theory, but HL3 would have to be a SteamOS exclusive in order for Steam Machines to take off at all... and if he's not a total moron, he won't do that. It will be released from Windows.
While I understand this buys people OPTIONS, most people are not technically inclined, nor will they see a benefit to getting a Steam Box. They have a Windows PC and they can play all their games -- on Steam -- on that. The Steam Box will offer them exactly the same capability, but less games, because games that don't work on Steam (ala Battlefield and Origin games) won't be available. Will SteamOS have a lot of games supported? Sure they will -- but they will still be supported in Windows as well. So why would you plunk down more money to buy a dedicated machine whose catalog of games can be played on a machine already?
Granted I do think there are some possibilities in terms of gaming via "big screen" and this can be resolved by MS releasing a similar set of software (RemoteFX comes to mind here as a technology) and possibly hardware to do exactly what the SteamOS can do around streaming games into the living room. SmartGlass is a platform that also could help here, and using the Xbox One as a medium to serve up PC games through the console interface via RemoteFX and Smartglass is actually doable and probably a good idea. Granted I won't put it past MS to fail to see the opportunity here, especially since that's the only component that SteamOS offers other than what is currently offered via Windows.
Time will tell, but SteamOS at this point isn't really interesting enough for me to move, and if I still have Steam on Windows, then I'll be just fine staying there. nVidia and ATI may provide driver support to Linux but realistically the performance is always better on the main platform -- Windows. I don't see that changing any time soon.