The first L4D DLC — dubbed the L4D Survival Pack -- is due for release this spring and introduces a new multiplayer game mode entitled, Survival, plus two complete campaigns for Versus Mode . 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. In addition, for PC gamers and aspiring developers, the first Left 4 Dead release for the Source Software Development Kit (Source SDK) will allow the creation of custom Left 4 Dead campaigns that will be discoverable via L4D's matchmaking system. The SDK update is also due for release this spring, and is free of charge to all owners of L4D on the PC.
I like the fact that they are releasing the SDK, I heard that because if the AI director, creating (good) maps for this game aren't as simple as they as usually are. So at least the user generated content will help bring more variety, but whether this helps make new AI Director friendly maps is another question.
I'm guessing the versus ones are the ones they didn't have on release. And do we know if this is coming before the Scout pack for Team Fortress?