I was reading a recent article in PC Gamer that explains the new survival mode coming to L4D on Apr 21, 2009. I will give a brief synopsis of what was said in the article.
The basic principle is that it is like the crescendo events that are currently in the campaign mode now but they never end and actually get harder. There will be 21 different events to choose from that are extensions of the existing events and one new map called Lighthouse. The PC Gamer folks stated that at first it was a normal crescendo event with more ammo and explosives but as time went on the number of special infected increased exponentially to where at one time there were 2 tanks, 3 hunters, and 2 smokers.
The object will be to stay alive the longest and the maps will all have leaderboards according to what I read. This sounds like it will be a blast and I can tell you that our L4D server will be running this mode as soon as it is available.