Valve recently announced that the next DLC pack for Left 4 Dead 2, Cold Stream, is set for release on July 24.
They are promising more details closer to launch, but they did reveal they are including a oft requested feature with the update accompanying the DLC. From now on: "All of the mutations will be available at all times. This will allow players to choose to match into games playing Taannkk!, Gib Fest, or even a user created mutation like Confogl."