According to the leaders of the Left 4 Dead 2 boycott—who recently enjoyed a trip to Valve—there's more downloadable content already in the works for the original Left 4 Dead, stuff beyond the already announced "Crash Course" episode.
According to on and off Left 4 Dead fan Walking_Target, Valve is planning something for release for the original, but wouldn't specify exactly what that was. It could be "a new Special Infected, a new Campaign or new Weapon(s)." Vague enough for you? "It's definitely one of the three, perhaps more," he writes, if that helps.
Walking_Target also indicates that Valve has plans to address some of the issues that plague the original, like stacking and camping in corners. Should Valve find a way to introduce one or more of Left 4 Dead 2's Special Infected—all seemingly designed to break up tightly guarded groups—that would certainly introduce a new dynamic to the game.
With "Crash Course" planned for a September release and Left 4 Dead 2 for November, we'd be surprised if the developer can sneak in more content for the original this year, especially when factoring in Valve Time. But they've surprised us before...
DLC and L4D Info [Steam via Shacknews]