I have never seen enough proof about the rumor about Senators cancelling flights but there were warnings about flying issued before 9/11 that did never reach the public.
The one story that bothers me most about post 9/11 is the one that tells of all flights in America being grounded while Bin Laden's relatives and other Saudis were allowed to be flown out of the country without interrogation by our government.
That I 100% believe true and to me I've always found it curious why it has not been a bigger story.
I can tell you that I remember on the news that a plane was going around the country collecting the Bin Ladins to take them out of the country. It said that they were respectable people with no connection and were being allowed to leave for their own safety. So it's not just an urban legend.