No Beckett's point is not valid and the reported actions of Atta and his crew do indeed have instances where they were fairly open in expressing their anti-American feelings, in insulting people, in being rude, and also where their actions were pretty weird -- like learning to fly a 747 but not how to land it.
While some few can live a double life easily, many are unable to all the time. Perhaps it is the term "sleeper agent" that is misleading. That implies years of formal training in spycraft, a formal selection and screening process, years of training in being a "sleeper". From all reports, Al Queada is much less formal, much less orgamized at that level of training.
How are you doing, by the way?