It certainly seems the case there will be a Valve blog for every part of Valve before there is a Half-life themed blog! You can now track the progress of Valve's linux team, currently 11 people strong, one of which we assume is Gabe.
The blog clarifies the build will be based on Ubuntu 12.04 and that Left 4 Dead 2 has already been ported successfully. The current goals are:
Only this week we discovered Sam Latinga joined the team to help along the project, which Valve will be keeping us informed on via the Valve Linux Blog.
- getting the Steam client onto Linux with full functionality
- optimizing a version of L4D2 running at a high frame rate with OpenGL
- porting additional Valve titles