Being made by Turtle Rock (not a huge track record admittedly, but they wrote the surprisingly competent CS Bot AI, and know how to make great looking models/maps) for the Source Engine.
The teaser looks like it's in engine, only with ginned up lighting and film effects.
"The world is populated in the main by people who should not exist." - George Bernard Shaw
I really hope there's co-op multiplayer. Or better yet, something along the lines of Zombie Panic. :-D
"Michael Booth: Left 4 Dead is a cooperative survivor horror game where four survivors must work together to escape a city teeming with an overwhelming number of the infected. If you work together, you have a chance. If you run off on your own, you will die. "
"The world is populated in the main by people who should not exist." - George Bernard Shaw
There was an article on this in...PC Gamer? I seem to remember it being 4x4 multiplayer co-op. 4 survival characters vs 4 zombie boss mobs, with AI zombie mobs filling out the action. Sounded cool as hell.