I read that at some point Steam was supposed to make a crossover map where the L4D2 survivors meet up with the L4D survivors. I think it'll be inserted in where the Dark Carnival map leaves off, because currently they leave in a helicopter and in the next map while they do mention they had to shoot the pilot, there's no indication of a helicopter anywhere and the level starts on a train, so the map they're making fills in that gap.
WRUP: Gonna be doing some progression tonight in ICC on WoW; we're currently 10 of 12 but the last 2 weeks we haven't had any time to try the newest boss we're at, so it'll be interesting.
It's going to be called "The Passing" and comes out either late this month or sometime next month. The original crew wont' be playable though you will meet them, three of them anyway. One of them is dead.
To further get L4D geeks excited they've leaked that more DLC will be coming out for L4D which will have a campaign where to complete it one of the players MUST sacrifice themselves so the rest can live. It's unclear at this point which original will be dead in canon but my money is on either Louis or Bill. I would lean towards Bill because the grizzled old vet seems to have that noble streak to him.