Valve's "Steam Box" will likely be shipping with a customized version of Debian/Ubuntu Linux at its heart. I've already written some early details about their Linux-based hardware gaming console for the living room based upon details I've known of within this article since talking with Gabe Newell last year at Valve about their Linux plans. For months I have been saying that their software platform will likely be based upon Ubuntu, likely "Ubuntu Core" and obviously an LTS release.
Anyhow, it was tipped off to Phoronix there's now a "hometest" Debian APT repository that has been public the past few days on the SteamPowered.com server. This repository is different from what's found in the public/default "steam" package repository.
Their known repository just has contained the Steam and Steam Launcher packages for Debian/Ubuntu-based distributions. What's of interest within this new "hometest" repository. The repository notes that this is for "Steam packages and updates for Steam based on Ubuntu 12.04.2 LTS."
Found within the package pool are experimental NVIDIA Linux graphics drivers, a Plymouth boot splash screen for Steam, and Valve wallpapers. There's also the Steam Linux client binary already offered to Linux gamers. A "steam-autoupdate" package is also present, which all it does is drop in a daily cron job of running the apt-get upgrade process in an automated manner.
Whether Valve ends up releasing their software platform as a "Steam Linux" distribution in its own right remains to be seen, but you should be able to achieve much the same effect by simply installing Steam on Ubuntu Linux. Expect most of the Steam software experience on the console to just be self-contained within Steam's "Big Picture" mode while being an open stack underneath for those Linux enthusiasts wishing to do some tinkering.