Valve's Chet Faliszek has said to PCGamesN that a prequel is not being made and the crossover is more like the Team Fortress 2 promotion items "but it's a little bit more involved." Full article here.
Chet has also confirmed it with Valvetime.net directly via email.
We already know that Overkill and Valve are collaborating on some sort of crossover between Payday: The Heist and Left 4 Dead but we don't know exactly what this "blend" of the two games will actually be. CVG are claiming that the crossover is a new game that takes place before the events of the original Left 4 Dead. There is no source given for this statement which is simply
In the mean time Valve have updated their in-game poll to ask Left 4 Dead 2 players if they will be attending PAX Prime this year. Perhaps we'll hear more info about this crossover there?
You can read the original story from CVG here and the relevant L4D blog post here.