The Left 4 Dead 2 mutation for owners of "The Passing" DLC for the next week is "Headshot!". In this mode, only headshots and decapitations will drop enemy zombies.
"Shoot them in the arm, shoot them in the leg, shoot them in the chest - it just doesn't matter! The only way these zombies are going down is with a head shot. Other hits will stumble them back, but only a head shot or decapitation will kill them." This mutation is available for the campaign.
Additionally, a patch has been applied to the PC version of the game via Valve's Steam digital distribution service. The Xbox 360 version will eventually see these changes implemented.
Valve describes the changes on the Left 4 Dead Blog:
While not removing Defibrillators, we have made some changes to them
