This week was the annual Animex, games and animation festival. One of the presentations held was by Doug Wood about the design and developmend of Left4Dead. I was lucky enough to be there with my camera so I wrote up an article on my website awsy.tk, with a couple of slides and some video footage of the original mod "Terror Strike" which the game was based on, as well an early source engine build. Here's a direct link:

http://sites.google.com/site/arewescaredyet/left-4-dead--feature-presentation

Cool stuff, keep up the good work.