http://store.steampowered.com/steamos/buildyourown
Figured this deserved its own topic here, despite being old news for most.
Quite limited in options for now, as well as in system requirements. NVIDIA only, and it requires UEFI support. Apparently it is possible to install it with BIOS, however. More on that here.
So basically it is debian which boots directly to Steam Big Picture.