The countdown on the Steam Living Room page has concluded, and this page has details on SteamOS, "available soon as a free operating system designed for the TV and the living room," the promised announcement about Valve's future plans for Linux and Steam. This is the first of three planned announcements on the topic, so expect more to come in about 48 hours when the next countdown concludes. Here's word:

As we’ve been working on bringing Steam to the living room, we’ve come to the conclusion that the environment best suited to delivering value to customers is an operating system built around Steam itself. SteamOS combines the rock-solid architecture of Linux with a gaming experience built for the big screen. It will be available soon as a free stand-alone operating system for living room machines.