The Survival Pack will add the new multiplayer mode, Survival, as well as two complete campaigns for the versus mode.
Valve will also release a "Critic's Choice" version of L4D at retail this spring, which will include all of the content from the original November release plus the Survival Pack material.
Good news for gamers as well as modders playing the PC version of L4D is the spring release of the free L4D version of the Source SDK, which will let modders create custom game campaigns that will be accessible via L4D's matchmaking system.
Referring to Valve's support of past games with updates and patches, Valve head Gabe Newell commented, "With Half-Life and Counter-Strike, and more recently Team Fortress 2, we've learned that we're no longer making stand-alone games but creating entertainment services."
With past Valve games, PC users have enjoyed substantial updates for free, however no pricing strategy was mentioned in a press statement regarding the Survival Pack.
Update: Due to a miscommunication, we had updated the original piece, stating that PC and Xbox 360 Survival Packs would be paid. However, Valve again contacted Edge to say that it hasn't ruled out the possibility of the price being "$0.00." Sorry for any confusion.