SteamOS Has Its Own Graphics Compositor
Aside from the Debian package repository mentioned in the earlier Phoronix article, more SteamOS details have been published to the SteamOS FAQ page.
Changes that Valve made to their Debian Wheezy base included backporting eglibc 2.17, adding updated graphics drivers, switching to the Linux 3.10 kernel, auto-updates from Valve SteamOS repositories, and they have written their own custom graphics compositor.
About this SteamOS compositor running atop X.Org, Valve's notes say, "Custom graphics compositor designed to provide a seamless transition between Steam, its games and the SteamOS system overlay."
Right now only NVIDIA graphics via the NVIDIA binary Linux driver are supported by the SteamOS 1.0 Beta.
In terms of why Valve went with Debian Wheezy over Ubuntu, the FAQ reads, "Building on top of the Debian core is the best way for Valve to deliver a fully custom SteamOS experience to our customers."
Stay tuned for more details. I'll be running many SteamOS benchmarks this weekend.
UPDATE: More details here. There's also now the first SteamOS benchmarks.