Moved here as this is about Linux
See also Valve's own page about SteamOS
. Some interesting bits from the related page about Steam Machines
Will I be able to build my own box to run SteamOS?
[Talking about Steam Machines] Can I hack this box? Run another OS? Change the hardware? Install my own software? Use it to build a robot?
Can I download the OS to try it out?
You will be able to download it (including the source code, if you're into that) but not yet.
As for the DE, I'd guess they have their own shell--at least that is what it looks like on the promo picture for the Steam Machines. To do away with as much as possible of the DE and give you a pure Steam experience.
BTW, check that second link and perhaps sign up to get a chance to be one of the 300 hardware beta testers; you'd get a Steam Machine for free!