Left 4 Dead DLC To Come Soon!

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Important stuff is bold

Solid details on the first batch of Left 4 Dead downloadable content should be out "shortly," according to Valve writer Chet Faliszek. But, in true Valve tradition, it appears the announcement has already been postponed, as it was expected "before Christmas."

Though details are light, Faliszek confirmed that the upcoming download will bring new scenarios, as "what we want to do is just deliver more content you can play at this point." These new scenarios will be released on the "[Xbox] 360 at the same time as the PC," instead of receiving a later Xbox 360 release a la Valve's Team Fortress 2.
Along with the "new stuff," the download will bring Versus support to the Dead Air and Death Toll campaigns, a move that will finally allow players to experience all four of the game's campaigns in the competitive mode.
Owners of the PC version can also expect the long-promised SDK at some point, which should allow for the creation of custom maps, scenarios and other mods.
"We should be announcing that before Christmas, what the DLC is," said Faliszek in a Kotaku video podcast, seemingly recorded December 15. "The holidays aren't actually so much delaying it as the press guys--[marketing VP] Doug [Lombardi]'s been taking some time. We'll have an announcement shortly, I don't know exactly when."
As for a movie editor that would let users capture and edit in-game footage--something Valve apparently hinted at in an E3 interview--Faliszek merely noted that the team wants to make sure that playable content will be "the first out--after that, I hate to speculate."
While pricing was not discussed, the new content is expected to be free for PC owners, continuing in Valve's tradition of providing PC gamers with free post-release support. However, it is uncertain if the Xbox 360 DLC will be free, as Microsoft is reportedly forcing Valve to charge for the upcoming Team Fortress 2 DLC on Xbox 360.​