Seems like the next update (out now on Steam, soon on the Xbox) will address two major issues with L4D.
-First, no more instant spawning during the finale so you don't have to worry about getting your boomer half way across the stage and being shot from a distance.
-Second, adding AI infected if your team has less than 4 humans.
http://kotaku.com/5465540/left-4-dea...ie-bots-are-go