Get the Big Picture has the clip, supposedly recorded on a cell phone during a Watchmen screening, and analyzes why it’s probably not the real deal, but a fan creation. Among other things, the story about how the recording was made just doesn’t add up.
I haven’t watched it with sound yet, but I’m inclined to agree.
Update: Thanks to Brandan, I’ve found a similar post at Slashfilm. No one knows who did make it so far, but it’s obviously not official.