Left 4 Dead 2 DLC "Dead Air" will be released early thanks to the gamers who answered Valve's call. Valve asked 20,000 gamers to earn the Stream Crossed achievement by this past Sunday night. They made this request on Wednesday, in just over four hours, 20,000 gamers already got the achievement. By Valve's deadline, that number has reached over 60,000. If you participated, give yourself a pat on the back. Thanks to you, Left 4 Dead 2 DLC Dead Air will be released on July 22, "at the latest," meaning if all goes well, it will be released even earlier.
Valve is still working on The Terminal and Finale maps.
Source: L4D Blog