A trailer for Payday: The Heist – No Mercy has just gone live and while no official announcement has accompanied it, it looks like it is a DLC map set in Left 4 Dead’s Mercy Hospital.
Developer Overkill Software tagged their YouTube video (which has now been removed) with the following:
“Have you ever wondered how the Left 4 Dead series began? It started with a heist!”
This appears to confirm what last month’s announced Overkill/Valve collaboration is all about.
More details when they become official.