The idea of a phased beta has been floated on both the WoW Insider Show, Legendary and The Instance. So, there's your rumor mill.

I don't think any of these shows has said "this is how it will work." The discussions have been more along the lines of "they might do it this way." However, just talking about it like that is enough to get people going- especially when that way of doing it is so incompatible with one's own thinking.

Personally, I don't think Blizzard would be able to get away with anything but a wide open invite to all Annual Pass subscribers. Whether they said it or not, the general feeling among the AP population is that beta will be open to everyone at the same time AND that it will function in much the same way as previous betas (i.e. NOT a series of "beta weekends"). If Blizzard goes another way, they run the significant risk of turning something that should be unabashedly good into a PR Nightmare.

Also, I think that Blizzard has numbers that indicate that most people who are invited to be in a beta end up doing little else than playing for 30 minutes to see a few things and say they did it and logging off to avoid spoilers.