If a photo of an internal Valve changelog list is anything to go by, we'll have a third installment of Left 4 Dead before we hear anything about that other Valve game with shooting from a first-person perspective. The image above was taken by a group of Dota 2 players lucky enough to tour Valve's offices recently as part of the International 3 Dota 2 tournament. If you look closely, you'll notice a reference to "L4D3" as well as "Source2", which appears to indicate that the game will use Valve's long in development new engine.
Here's an image which shows more clearly the reference (via NeoGAF ):
Of course, news that Valve is developing Source Engine 2 isn't new: Gabe Newell reportedly told a tour group last year that the studio had "been working on new engine stuff for a while," and that they were just waiting for a game to roll it out with. Could Left 4 Dead 3 be it?