According to a Microsoft press release issued this morning, Left 4 Dead's Survival Pack DLC will be released on April 21. This release concerned the Xbox 360 version but Valve traditionally releases its PC content in a timely manner, and has previously stated that it will come by April 21.
The free Survival Pack adds the new Survival game mode, where players are given time to dig in before fending off unceasing zombie hordes in an attempt to survive as long as possible. Survival Mode is played on 12 new maps--11 being sections of existing levels and 1 created especially for Survival Mode, Lighthouse.
The Survival Pack also brings Versus mode to the final two co-op campaigns, Dead Air and Death Toll. This release coincides with the release of Left 4 Dead: Critics Choice Edition, which contains the original game plus the Survival Pack.
Valve has been contacted for confirmation of the PC release date.