After over a year of play testing, Valve announced on its official L4D blog that it will finally be releasing their new downloadable content for PC and Xbox 360. Those new add-ons they were testing include the Cold Stream campaign for the PC and Blood Harvest, Cold Stream, Crash Course, Dead Air, and Death Toll for the Xbox 360.
Valve was also nice enough to decide to deliver on a request many have been pleading for.With the update going out at the same time (also for the PC), 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.
They also mentioned some "surprises" that will be announced at a later time. No price has been set for the DLC.