As much as I enjoy L4D, the matter has indeed been discussed before. I do appreciate your interest, as it would be a fun game. However, polls have been made, and previous discussions have all but ceased the creation of an official division.
For now, I believe that L4D is simply an unofficial division in which is being supported by eGO (just look at the nice servers they just provided us) just like how PVKII was an unofficial division (and project) in early 2008. Personally, the server selection is the only thing that is preventing it from being a true contender of being a realistic division. Otherwise, I think it would have been entirely supported by now.
For now, the thread will be locked, as the discussion of the division has happened. If anything arises, I'll let you know. If you have any questions, feel free to PM me, and I'll address it to Council. Thank you.
