It will pack new singleplayer, multiplayer and co-op campaigns. New Survival, Versus and Scavenge arenas too. It's out in early spring.
We more or less knew all this anyway but X10's presentation and demos of The Passing means some new juicy media will be flowing onto the Internets real soon. Have you been enjoying Valve's latest Left 4 Dead antics with its sequel, videogamer?