I honestly don't think Apple ever does something accidentally related to marketing. Everything is planned and executed to perfection. You can say a lot about Apple, but they're very good at marketing and "accidentally" stealing attention (remember that guy "leaving" his unreleased iPhone in a bar?).
Jobs told on stage that FaceTime was going to be an open standard, and anyone could connect to it. Granted, WWDC is a developer conference but a WWDC keynote is still very much a marketing event.
As much as a portion of my mind wants to think this leak was a well timed attempt to draw tablet news in their direction mere hours after Google released information about theirs, I honestly believe it was just a mistake. Apple gains nothing by releasing this information a mere 24 hours before their big announcement. This is just a mistimed button click.