Earlier in the month, Valve revealed that the Left 4 Dead downloadable content would be released no later than April 21st. According to a Microsoft press release, that's the exact date it will be available.

The press release touted the L4D Survival Pack as one of many pieces of DLC coming to Xbox Live. It's not clear if the PC version of the game will get the Survival Pack on that date as well. The upcoming map packs for Halo 3 (April 9th) and Gears of War 2 (March 31st) and episode for Fallout 3 (March 24th) were also mentioned but all those release dates were already confirmed to my knowledge so L4D gets the headline love.

The L4D Survival Pack adds a new Survival mode with twelve maps as well as Versus mode support to the Death Toll and Dead Air retail campaigns. You can read more details about the Survival mode here.