More info starting to come in:
"A senior government official, speaking on condition of anonymity, told NBC News that a crew member noticed a man reading a flight manual and notified the pilot. "
http://www.msnbc.msn.com/id/11909506/
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It may have been a rehearsal -- we know the 9-11 hijackers did "dry runs" too.
I always wanted to take "How to smuggle anything" as reading material, or an empty parachute pack as carry-on (pre-911), but common sense and not being able to get an old parachute prevented me.