I haven't seen this, but will look for it. Also, my friend goes to a film class and saw United 93 before its premiere. He gave it 4 out of 4 stars and his average rank on movies is 2 1/2.
Technically, the conspiracy theory has some legs since I believe there were orders issued to shootdown any plane that did not identify itself. I'd be surprised if it were true considering the timing of all events - they would have had to get that fighter jet up in the air and in the right place pretty damn quick. I don't think there was enough time.
My boss was in a plane when everything was going on - Newark, NJ to LAX. He said they had just gotten their drinks and he was just getting back into his book when the pilot announced they had to land. The flight attendants practically grabbed things out of peoples' hands. He was fortunate to share a ride and drive back. His boss was already in LA and got stuck there for a week.