If you and your three favorite undead-massacring accomplices are already strategizing in anticipation of next week's release of Left 4 Dead's punishing Survival Mode, you might want to turn your attention to a recent post on the L4D blog. Two of the game's developers have broken down the zombie spawning mechanics of the new mode, explaining the steps they took to slowly ramp up difficulty over time while avoiding harsh difficulty spikes. We still consider the arrival of a Tank a "harsh difficulty spike," but to each his own.
It's worth checking out, if only to see what goes into the development of the beloved AI Director. It's like watching a baby grow! A sadistic, hateful baby.