Why would MS care?! A third party creating hardware that runs MS OSes and links to MS services is their ideal. If the whole world created Xboxes without MS having to, they'd be much, much happier. I'm sure if any hardware manufacturer wants to create a gaming PC, MS will happily charge them to use their OS and tools and ensure content is shared with Live! with a cut.What would the Valve box use for OS and API? Linux and openGL? I don't know if you can lock down windows to that level without having access to the source code, and MS surely won't give access to that.
But it's for this reason no-one like Valve will create a box. The money to cover the hardware comes from the software. Consoles only survive (and let's be realistic here - how many billions have been lost by companies from one generation to the next?! Consoles are hardly a license to print money unless you're lucky). Why would Valve lead with PC ahrdware to play games when instead they can leave people to buy a PC (more useful) and just buy into Steam?