Please, let's stop speculating about what happened onboard the aircrafts, the whole thing is terrible enough. Maybe there will be some light on that matter when they find the black boxes and cockpit voice recorders, analyse them and release data to the media.
Passengers overthrowing hijackers only works in movies, in my opinion.
The question is, how was it possible to hijack four (!) planes in the USA at the same time, from the two biggest airlines?