Valve's Kerry Davis and Scott Dalton have posted a rather detailed look at the upcoming Survival Pack for Left 4 Dead (Xbox 360, PC), exploring a lot of the nuances found in the new Survival mode. The name itself triggers images of having to survive for as long as possible, but it looks to be more than that.
First, the devs expect that rounds will last only about ten minutes. That may seem short to some of you, but the "extreme pace" of the Survival mode will often result in the number of zombies killed in a single round outnumbering that in an entire campaign.
It's also quite possible to go over the projected ten-minute round, but you better be prepared for it. As shown in the graph below, spawns will become closer and closer together the longer the round is. At around 15 minutes into the round, you'll end up facing a newly-spawned Tank every 10 seconds or so.
There's still a lot of details, most of them about the gameplay considerations that went into Survival mode, on the Left 4 Dead blog. It's an interesting read, so check it out by clicking on the source link below. The Survival Pack will go live next week.