Valve have issues a press release today that says that a new game mode, Versus Maps and the SDK will be released this Spring!
The first L4D DLC is dubbed the "Survival Pack" contains the new Survival game mode and two complete campaigns for Versus mode. The campaings are not specified but it is assumed that they will be Dead Air and Death Toll.
The SDK will also be made available for the PC in the Spring!
There are no details yet as to how Survival Mode will play.
Spring looks to be a good time for killing Zombies!