"Due for release this spring, the DLC for 2008's best-selling new game property on the PC and Xbox 360, is dubbed the L4D Survival Pack and introduces a new multiplayer game mode entitled, Survival, plus two complete campaigns for Versus Mode (Death Toll, Dead Air). A Critic's Choice Edition of the game will be heading to retail stores this spring, and will include access to all the content introduced in the L4D: Survival Pack."

Source: store.steampowered.com/news/2234/

Well I'm only looking forward to the new survival game mode, wonder what that'll be like. I already play on Death Toll and Dead Air with friends on a private server, so that's nothing new for me =P
There should also be a map creator thing being released around the same time, and if everyone could join on custom maps made by other players, then there's really something to be looking forward to =P