So this video got posted and then removed pretty quickly, but a few outlets managed to snag it.
Overkill’s PAYDAY and Valve’s Left 4 Dead crossover game will (at least) be in the form of a new map, with what looks like some new weapons and effects (darkness and flashlights). This is exciting stuff, coming from someone who plays PAYDAY damn near religiously. Seriously, check this out and tell us it’s not awesome.
Badass. No other information… yet.
Source(thanks to GameGravy member e-z-e for the tip!)