We've told you before that Overkill Software, creators of PAYDAY: The Heist, had a surprise which included some sort of collaboration with Valve set in the Left 4 Dead universe.  It's now been leaked as to exactly what that is.  It's called PAYDAY: No Mercy and looks to be a map or a set of maps for PAYDAY: The Heist set in the familiar Left 4 Dead level No Mercy.  But that's not the interesting part for the four player co-op game.