It's all go at Payday: The Heist developer Overkill Software these days. As well as working on a sequel and DLC for its coop crime caper FPS, and being acquired by Starbreeze Studios, the Swedes have revealed they're working with Valve on something Left 4 Dead-y.
"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."
As for what this'll actually be, who knows? Payday clearly took a few hints from Valve's zombie 'em up, so it wouldn't be too jarring for the worlds to collide. Valve characters often make appearances in other games, but that probably wouldn't meet the "in-depth" requirement. Might that mention of the hospital be a cheeky nod to Left 4 Dead's 'No Mercy' campaign, set around an overrun hospital?
The original group of Starbreeze has three projects in the works itself.