In a presentation at Ubuntu Developer Summit currently going on in Denmark, Drew Bliss from Valve said that Linux is becoming more viable platform as an alternative to Windows 8. Windows 8 ships with its own app store and it is moving away from an open platform model.

Some other points from the talk:

Steam client is running nicely on Ubuntu and many developers have approached them with good game products.
Cooperation with Canonical has been good.
Ubuntu is preferred platform as it has a large user base and good community support with a strong company like Canonical behind it.
Linux has everything they need: good OpenGL, pulseaudio, OpenAL and input support.
New Source engine games will be available for Linux.
No firm time frame for Steam Linux release, but soon.
Copy protection is up to the game publishers.

Drew announced that anyone with a Launchpad account attending UDS will be given Steam access through a beta key.