The security question needs to be addressed at the many "lines of defence".
As has been said before, Airline Security begins at the point of check-in. Vetting may be incorporated at the booking stage but as previously noted, a large database would be needed and may not help with "clean" terrorist operatives.
Check-in security must be much stricter. Any kind of knife, pen-knife or blade must be carried in checked in luggage (carried in inacessible holds) or confiscated at the x-ray/security/customs. Maybe further x-rays/frisk searches at the jet bridge or gate.
As for carrying AirMarshalls, the deterrent element would be there. If someone knows an AirMarshall is carried on a particular airline, maybe there is less chance of someone attempting to take over the aircraft. SkyMarshalls should not be a "visible" deterrent, being in uniform would single that person out for the first attack/incapacitation. (I'm guessing a "sleeper" is non-visible). By introducing a firearm into a secure environment you are providing the attacker with a means of threatening the crew/passengers. Sounds dodgy to me.
Maybe it would be possible to carry prefilled, one-shot syringes, containing an incapacitation drug, concealed at various points in the aircraft (under seats/in galleys/on the flight deck). This would require being in close quarters but could be administered to the arm, leg or hand (maybe while you are keeping your head down in the isle seat).
The last line of defence is actually the flight deck door and not the flight deck crew. If the attacker gets the flight deck door open then you are already in trouble. the door does not need to be fully open to point a gun barrel inside!
There will no doubt be many suggestions on how security can be improved. We should all hope that the powers that be look into it as much, if not more intensely as we PPruners.
Clang, clang...the sound of stable doors closing!
My thoughts and prayers to all those affected by these tragic events, especially those of the FDNY who lost their lives going in to help...
"When I'm called to duty God, wherever flames may rage, give me the strength to save a life, whatever be their age...."(Firemans Prayer)
[ 15 September 2001: Message edited by: Top Loadie ]