I think many of you are forgetting this is just a prototype. Straight from the information on Steam.
"If you guys are delivering an OS to hardware manufacturers, why is Valve also making its own box?
We're conducting a beta of the overall Steam living-room experience, so we needed to build prototype hardware on which to run tests. At Valve we always rely on real-world testing as part of our design process. The specific machine we're testing is designed for users who want the most control possible over their hardware. Other boxes will optimize for size, price, quietness, or other factors."