Payday: The Heist developer Overkill Software confirmed that the studio has been working with Valve on an "in-depth crossover" between their game and Left 4 Dead.
The game, which I'm going to assume is called "Zombies and Robbers" until someone says otherwise, is already in production, according to the Overkill website.
“As perceptive gamers will have noticed, several hints have recently been dropped into PAYDAY The Heist, which has led to various rumors," said Overkill director Ulf Andersson. "We are excited to be able to confirm that an in-depth collaboration between OVERKILL and Valve is currently in production."
"We are working on a very cool blend of PAYDAY and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done.”
Overkill is currently in the process of being purchased by Syndicate developer Starbreeze, who bought the studio to take control of the Payday license, among other things.
Overkill says fans should "keep an eye out", so it's entirely possible that we'll be hearing more about "Zombies and Robbers", or whatever Valve and Overkill decide to call it, very soon.