Left 4 Dead, "shortly." According to a video podcast with Kotaku, Chet Faliszek, one of the game's writers, confirmed that the team was looking to add Versus functionality to the Dead Air and Death Toll campaigns and to release brand new scenarios.
Faliszek also stated that, unlike Team Fortress 2, Valve plans to release DLC for the Xbox 360 and PC at the same time. During the recording, which took place on December 15, Faliszek speculated that DLC news would be releases before Christmas Day but said the announcement could be delayed due to the press team going on vacation. "We'll have an announcement shortly, I don't know exactly when."
No pricing details for either platforms were discussed.
[Thanks to everyone who sent this in. Via IGN]