Will be right around July 22nd or a little later
Valve issued a challenge to Left 4 Dead 2 players: if the community could earn the Stream Cross achievement 20,000 times, Valve would release the Cold Stream DLC pack for L4D2 early.
The company was apparently surprised, but the L4D2 community was able to earn this achievement 20,000 times in about 4 hours. In fact, in 24 hours they passed 60,000+ and locked in the early release.
Valve says that the the testing of the Cold Stream DLC pack has been going well and players continue to test it and submit bugs. Valve is addressing the bug reports and feedback as quickly as it can, but it would appear that they will not be able to release it as early as expected. The release target is on or about July 22nd, but it could end up being a little after that.