Valve has been up front about wanting to get players involved with testing the Cold Stream campaign for Left 4 Dead 2 on Steam. As an extension of this, the studio has issued a challenge to the community, complete with its own reward -- early access to downloadable content.
20,000 people need to survive a campaign of Cold Stream on any difficulty (see: the Stream Crosser achievement) by the end of Sunday. Doing so will result in Dead Air going live for all Steam players on July 22. Speaking of which, Left 4 Dead 2 is on sale for $4.99 today.