Despite some vague-ish comments from Valve that this wasn't the case, a video turned up on the Payday: The Heist YouTube channel indicating that Valve is working in some capacity with PayDay developers Overkill on something called No Mercy, which will explore the origins of the zombie virus from Left 4 Dead. The video was quickly pulled from the channel, but was then grabbed by IGN and subsequently posted to youtube by several other users.
Update: Valve's Chet Faliszek has emailed us, saying that the YouTube video isn't a prequel, that it was "just some over-excited marketing guys."
"This is a 100% Overkill made mission for Payday: Heist simply set in no mercy hospital from the Left 4 Dead series. It is not telling the origins of the Infection and is not canon. There is a little cameo from one of the L4D characters (the comic should give you a hint) and a fun little easter egg with a payoff later in Left 4 Dead. Just us having some fun with a studio we are friends with and who have also made a great co-op game."
Before it was pulled, the YouTube description said, "Have you ever wondered how the Left 4 Dead series began? It started with a heist!" The level looks like a heist gone wrong (or at least, gone violent), and before long things have gotten hairy. Left 4 Dead flashlight-shooting ensues, and I gotta say it's weird to see the things at the other end of the flashlight beams shooting back.