A Microsoft press statement released this morning revealed that the Xbox 360 version of Left 4 Dead is set to receive the new downloadable content on 21st April. While there's no word on the PC version (obviously, as it's a Press Release made by Microsoft...) it's safe to say that due to all the extra red tape the DLC has to go through on the Xbox version, the Survival Pack will hit the PC no later than that date.
On top of this, as Valve have previously stated that they intend the downloadable content to hit both platforms simultaneously, it seems reasonable to assume that this will be the date for the PC version as well.
Via CVG.











