Anything that tries to disrupt, or at least compete on the same level with the Xbox and Playstation, is a good thing, and that includes OUYA as well.
It seems like somebody like Alienware, or even just somebody on Kickstarter, could put together something impressive at a XBox720/PS4 pricepoint.
Otherwise I can't see an impediment. Maybe Valve could even have a 'designed for Steam' certification program or something.
The MistBox ;)
I don't know, though. To be honest, I'm kind of shocked no one has tried this yet. It seems like such an obvious winner, even if there is a slight chance you would go to court with Valve.
I think it's a great idea!
Now that's a Steambox.