In this article Homeland Insecurity from The Atlantic Online Bruce Schneier says that for preventing highjacking "The only ideas I've heard that make any sense are reinforcing the cockpit door and getting the passengers to fight back." Along these lines, what about giving airline passengers rubber truncheons? Put them in pockets on the back of the seats. Wire them so that if they are removed an alarm sounds in the cockpit. A bunch of passengers with truncheons could probably overwhelm some hijackers before they could get through the cockpit doors.