A write-up on the L4D2 Boycott Forums has the impressions from the visit to Valve by the founders of the L4D2 Boycott Group (thanks Dave). Included is an outline of future DLC plans for the original Left 4 Dead:

Now for some good news: There is more DLC for Left 4 Dead coming after CrashCourse. We were given a rough idea of what it should be, however we were asked not to talk too much about it so it didn’t “influence the creative process”. I can tell you this: it’ll either be a new Special Infected, a new Campaign or new Weapon(s). It’s definitely one of the three, perhaps more. This is pretty much the only thing that we were asked not to talk about (remember, *asked* we didn’t sign anything). To that end, we’ll continue to bug Chet about this after Crash Course drops.

I can tell you guys this though; Chet and the rest of the L4D team are still excited about the original four survivors and working the stories of L4D and L4D2 together. To that end, Gabe told us that they’re working on a project that will bridge the stories of L4D and L4D2 and that it will be available through steam (I believe the word ‘free’ was tossed in there too, just don’t quote me on it). It was also hinted at that they were doing something for the Mod community; Agent of Chaos heard more about that though.