The matter's settled. The DLC for Left 4 Dead will be absomolutely free on PC and 360. Valve have proclaimed the good news about the L4D Survival Pack, which we describe in words here. Quite how Valve have managed to get something out on 360 without forcing the masses to tithe to the Redmond Beasts we don't know, but that's not our problem anyway.
The Survival Pack will include the mysterious "Survival" multiplayer mode (I'm going to take a guess - it's a mode where you see how long you can survive for, without safe rooms?), as well as Versus modes for Death Toll and Dead Air. And of course the PC version will come with the much-awaited SDK gubbins. Carry on.