Update: Sorry, folks, but mere minutes after our story went live, Valve confirmed that the teaser is in fact a fake. It's still pretty cool though. You can read my our initial thoughts on the presumed-fakeness of the video in the original story below:
Original story: A very impressive teaser that turned up today on YouTube has a lot of people wondering if a new Left 4 Dead game has suddenly sprung a big leak, or if Valve is perhaps trying its hand at viral marketing. It could be—Valve did recently tease a new "flagship" game coming this year, after all, and this wouldn't be the first time we've heard whispers of Left 4 Dead 3—but the far greater likelihood, sorry to say, is that it's a fake.
For one thing, the video is complete and polished but purely environmental, which isn't at all how Valve rolls: Left 4 Dead is frantic and visceral (check out this L4D2 teaser for evidence of that), and this, for all the atmosphere it stokes, is not. The zombie hand at the end is suspicious as well, because it's a direct copy of the original L4D hand rather than something indicating the third game in the series. Joke about Valve's inability to get to "three" if you will, but I can't see it breaking an established pattern on a major game franchise just to nod-and-wink at a silly inside joke.
I think the presence of real-world products in the game, like Pepsi and Life, also points toward fakeness. You can't just throw stuff like that into your game or your promo materials without some sort of licensing deal, and there's no evidence of that—and I don't see Valve doing that sort of thing in one of its game worlds anyway.
So it's possible that this could be a surprise L4D3 leak, but only very, very (I mean, very) slightly possible. We've reached out to Valve to ask, just in case, but I'm confident enough to call it now: Very nicely done, yes, but also very fake.