Earlier tonight, Left 4 Dead's Survivial Pack arrived for download for Xbox 360 users. The DLC is 101.18 MB and is now playable via an in-game update and can also be downloaded through the Xbox Live Marketplace.
The free update brings the new Survival mode to the game as well unlocking "Death Toll and "Dead Air" campaigns for Versus mode (online multiplayer).
The PC version of the update arrives later today.