We've all been waiting for it, and I think we're finally going to get it.
The Left 4 Dead downloadable content "Survival Pack" will be coming on or before April 21st. A Destructoid article specifies that it will be made available on April 21st, whereas a different article at N4G.com claims that the release date will be before then, prior to the release of the Critic's Choice Edition.
As per prior announcements, the Downloadable Content will be free for both PC and Xbox 360 users, continuing Valve's proud tradition of maintaining and expanding on their products for free for gamers.
The Survival Pack will contain a new "Survival Mode" in which players are required to fend off numerous waves of zombie attacks, and will also bring Versus Multiplayer to the two campaigns which lacked it in the originally shipped product. No word on new campaigns.
If you want to get your group back into the game, to warm up for the DLC, I would encourage any and all PC L4Ders to visit L4Dmods.com and L4Dmaps.com, for their user-made mods and maps, respectively. L4Dmaps in particular already has several playable campaigns-in-progress as well as some completed ones. If you'd like my suggestions, check out Orange (a complete campaign which lacks textures, but immerses the player in a broad array of different and unusual scenarios), as well as Zelda (a faithful recreation of a Zelda dungeon with zombies, and a downright incredible boss battle that you will not be able to predict). With the fast-approaching introduction of the SDK, soon more mods and maps of even higher quality will be available to us, so if you haven't memorized or bookmarked those links yet, you may want to do so now.