"During a hands-on session at Microsoft's E3 booth, Valve programmer Kerry Davis was more than happy to talk about the past, present and future of Left 4 Dead. The most notable piece of information came about when I asked him about any possible connection between the two games.
You won't see any familiar faces from the first L4D and there isn't an interwoven plot between the two games. But, the original L4D's campaigns, on the other hand, could make a reappearance.
"There's a good chance that, at some point, you'll be able to play the old maps in Left 4 Dead 2," Davis teased."