Was wondering when we were finally going to see some more Left 4 Dead content rather it be a new title or DLC for L4D2 and managed to dig up a couple of articals. Anyways with a still "pending" release date a new Left 4 Dead 2 DLC called The Cold Stream is supposed to be coming to PC and Xbox 360 sometime later this summer, which will include the new Campaign of course as well as the 3 other campaigns from the original Left 4 Dead to play on the sequal much like "No Mercy" that was included on The Sacrifice DLC. Latest info on this is stated here. Also if you are PC gamer for Left 4 Dead you can now recieve the Dead Air Beta DLC on Left 4 Dead 2 right now.
If you are more interested in seeing some bits of what the Cold Stream Campaign is going to look like go ahead and check this
link out. its a bit old but has come PC screenshots and videos of the new campaign!