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...