Last month, Valve offered to unlock the Left 4 Dead campaign Dead Air in Left 4 Dead 2 earlier than had been previously announced if 20,000 fans stepped up and played through the still-in-beta DLC campaign Cold Stream. It was a successful promotion, so much so that it's being repeated for the next DLC campaign from the original L4D.
Valve revealed in a blog post that Blood Harvest's release date will be determined by the fans, with the beta version unlocking on "the next weekday" after 60,000 people unlock the Connecting Flights achievement, which is earned by playing through the entire Dead Air campaign in Versus mode. The post notes that "60,000 will be hard," but more than that number turned out for the last challenge when only 20,000 were needed. Methinks the fans have this one in the bag.
Source: Left 4 Dead Blog