The development of the Cold Stream DLC for Left 4 Dead 2 is almost finished, however there is no release date announced yet.
According to an update posted on the official Left 4 Dead Blog, work on Cold Stream is now focused on making small tweaks and squashing bugs.
“Now that we have a solid version of the maps (some minor changes still coming), we are focused on the DLC and getting some pesky bugs out of the shipping version. As soon as we have a release date we will let you know.”
The DLC consists a community made campaign along with some new maps, and a new melee weapon.