I don't know but I think Valve like to take a backed off stance and let other people get on with it, to be the facilitators behind the scenes, the glue that holds everything together kinda thing.
But, it's a bad way to handle marketing and PR strategy, bombarding people with so many units from so many vendors all at once for sure.
I think Valve will want it that any individual or any company can install Steam OS to a wide range of ready built PCs, self-builds or things PCs they already have but like Timey says, if it's going to be in your lounge most people will want it to look good under the telly.
I expect there will be loads of issues for a white, it's a computer afterall. I guess it's gonna depend on driver support from nVidia and AMD for SteamOS. It's a good move though, I feel, and Valve put a lot of work in and have a good track record of ensuring things aren't half baked on release so problems are minor rather than major catastrophes.
The Valve control pad has me interested and I can imagine support for the Occulus Rift VR headset coming so that's quite exciting really.