Steam might be running server maintenance or upgrades. Have you tried starting the game directly? I.e. not via Steam
Truthfully, I didn't think you could. So I went right to the DVD and hit Play, but I got the same error(?) message. I don't like to call it an error, as it's not. They just won't let me play "right now". Even if they had said, "Sorry, due to maintenance" etc. I wouldn't be so pissed off. But just to tell me it's unavailable, and for me to "try at another time" really roasts me. What "other time" would they like me to try?
Oh, and just to prevent the "did you do this" questions, I did update the drivers.