As an update to the previously cited release date
of April 21st, Valve has specified that the Left 4 Dead downloadable content will be made available for Xbox 360 users at roughly 2AM PST, with the PC version slated to be released later in the day. The free DLC pack will bring versus mode campaigns for “Death Toll” and “Dead Air”, as well as a new game option, “Survival Mode”.
Survival Mode is said to offer a faster-paced and shorter-lived gameplay to the traditional campaign by pitting players against increasingly frequent hordes of zombies on 12 specialized maps. During testing, Valve’s employees were unable to endure the action beyond 9 minutes, leaving them to surmise that the number of undead killed in a single round of Survival Mode often outnumbers an entire normal campaign.