Valve are the masters when it comes to incentives to play games; there was that whole potato sack deal for the release of Portal 2 and now they are offering Left 4 Dead 2 gamers the chance to unlock the new DLC early.
How do you help the cause? Valve want you to play through the Cold Stream campaign. If 20,000 people complete the campaign by Sunday (assuming it will be Monday for Aussie folk), then they'll release the Dead Air DLC early.
Coincidently, Left 4 Dead 2 is now on sale for under $5 for today only. Check out our Summer Camp Sale roundup.