Steam used to block all downloads as soon as you launched a game. However, there is no need for a workaround anymore, thanks to the new content system: when you download a game, it is possible to launch and play a single player game under Steam and continue downloading others games in the meanwhile.
Fixed downloading new update while game is still running
Reference: Steam client updates.
In addition, the new content system will allow us to build several new features that we’ve often heard requested. Upcoming client releases will include things like download scheduling, bandwidth throttling, and prioritizing which games get downloaded first. You’ll also be able to download an update to a game while you’re playing that game; Steam will apply the update after you exit the game.
Reference: Steam news.
Some people are talking about downloads being suspended, but I have never experienced this problem. I suggest choosing another server and trying again if this happens to you.
Apparently, a new feature is coming.