A press release issued by Microsoft this morning gave a firm release date for the Left 4 Dead downloadable content. April 21 is the date that every L4D fan should get excited for and take note of.
This announcement was only for the Xbox 360 version, but it seems like a safe assumption to make that the new content will also be made available for the PC version at or very close to this date. Obviously, this isn't confirmed but it does seem like a safe bet.
The content that is expected to be released on the 21st will include Versus mode versions of Death Toll and Dead Air, the Survival Mode, and possibly the inclusion of the SDK for PC owners. The special release of Left 4 Dead: Critic's Choice Edition is also slated to arrive this Spring and will include all of the Survival Pack content.