Wow - This teaser is attached to TRANSFORMERS. It's a hush-hush movie from JJ Abrams, creator of Alias, MI3, etc, and is operating under the title Cloverfield or its release date 1/18/08. Check it out here.
It's like some odd/awesome combo of Blair Witch (shot on "home" video) and Godzilla (or that Korean flick The Host). Sweet.