Is that from the private beta, or did I miss an announcement regarding wider distribution?

The Linux client is still in private beta, but you can easily circumvent it by just downloading the .deb and installing/running it on Linux anyway. FWIW, I wasn't invited to the beta right away, but about a week after I started running it anyway, I was invited.

The only issue you might run into is that it'll sometimes say your account is not authorized to use the Linux beta, but there are directions online on how to get around this by adding some flags and launching it from the CLI.

I'm not sure why I bothered though, since my Linux ultrabook can barely run any games anyway.

I did install the .deb, but when launching the client I get a message that I am not invited to the beta yet and that I cannot use this - then closes. Any work-around ?

Run "steam steam://open/games" from the CLI to launch it instead. Try a couple times if it doesn't work the first time.

http://www.omgubuntu.co.uk/2012/11/reddit-users-bypass-valve...

Valve did find OpenGL outperforming Direct3D with their engine.

http://blogs.valvesoftware.com/linux/faster-zombies/

Did you do any FPS comparison ? What graphic card are you using? I would be interested to know more about your experience.