People always seem to find someway to sneak some footage during Quakecon and other places that ban footage. I'm just fine with just reading the newly released information though.
I remember seeing off screen New Vegas footage from QuakeCon last year. That game had a public playable demo though, Skyrim is being presented by Todd.
What we'll be seeing a lot of unfortunately are people who will post a lot of false info saying that they were at QuakeCon and saw the presentation. With Gstaff attending QuakeCon, we'll probably have to wait a few days for him to confirm what's true and what isn't, since he'll be busy at the event.