This kind of came out of no where.

http://kotaku.com/5520780/left-4-dea...game-mutations

It's official: Left 4 Dead 2's The Passing will be out on Thursday, April 22 for the Xbox 360 and PC
Purchased being a relative term, of course. Xbox 360 players can expect to pay 560 Microsoft points ($7) for The Passing, while PC players can purchase it for completely free, which I suppose doesn't count as purchasing it at all.