The free Left 4 Dead "Survival Pack" expansion is slated to hit next week. The Xbox 360 and PC add-on officially makes two more campaigns playable in Versus mode, adding another all-new gameplay mode, Survival.
Frankly, I'm a little vague on what Survival mode brings to the table, but Valve does its best to explain the new addition on the official Left 4 Dead blog. Two interesting key points about the mode is that gameplay sessions are expected to last about ten minutes (or less) and that the "extreme pace" of the mode, "the number of zombies killed in a single round often outnumbers an entire campaign."
Valve's Kerry Davis and Scott Dalton go into great detail about the pacing of the Survival mode, explaining how the pacing differs from the regular Co-op and Versus modes, noting that the regular rush of Hordes, Special Infected and Tanks will ensure that every reload, every revive of your fellow Survivors will have to be perfectly timed.
The accompanying helpful chart shows some frightening data, with games lasting beyond the expected ten minute length spawning new Tanks every 45 seconds or so. At fifteen minutes, it's less than every 10 seconds.
Surviving the L4D Survival Pack [Left 4 Dead Blog]