It's no Half Life 3, but at least it's something.
A DotA 2 player visiting Valve's offices for a sneak preview of The International 3 managed to catch a picture of an internal changelog, which shows evidence that the developer's been working on the Source 2 Engine and Left 4 Dead 3.
While it's not an official confirmation from Valve that Left 4 Dead 3 is official, this leak allegedly comes right from the source (no pun intended.) Still, with no release date, it's possible that we may be waiting for a long time.
You can take a look for yourself via NeoGAF.