While Valve has yet to announce a release date for Left 4 Dead's upcoming downloadable content, they've revealed that it will be released before the Critic's Choice Edition of the game (which includes the full retail version as well as the new content) arrives in stores on April 21st. That's right, there's now a second reason to look forward to 4/20.

The downloadable content will add Versus mode support to the two retail campaigns currently lacking it (Death Toll and Dead Air) and a new Survival mode with 12 maps. Most of the new maps are, as we expected, small pieces broken off from the existing maps. There is one new map, though, set in a lighthouse.

Valve revealed a few more details on Survival Mode to IGN yesterday. At the beginning of a match, players gather up equipment and get into position before then alerting the Horde (by flipping a switch or whatever, much like a few moments from the retail campaigns). Then, waves upon waves of zombies will descend upon you. To make things really interesting, you'll even get multiple boss zombies attacking. It's possible that the game could even throw two Tanks at you.

The objective of Survival Mode is to survive as long as possible - there ain't no escape. The record for survival among Valve employees is apparently nine minutes. Sounds like a quick and dirty alternative to the 45 minute-1 hour long campaigns. As previously announced, the DLC will be free for both the Xbox 360 and PC versions.