The whole issue of arming anyone on a plane makes me shudder.
Even if a sky marshal is available, the chances of them being able to make an accurate shot and disable anyone who is a possible threat is low - and these are the guys who have regular practice with guns.
As for stun guns/tasers - forget it! The accuracy of these devices is even less predictable, regardless of who is operating them.
Previous hijack rescue missions on the ground have proved on several occasions that the use of guns on aircraft is highly risky, never mind airborne occurances.
It has been readily accepted that the perpatrators of the Sept 11th atrocities had not informed their fellow hijackers of the true mission intentions, and there is some considerable doubt as to whether they terrorists would have gone through with the hijack had they known.
Improvements need to be concentrated on security on the ground, not hysterical soloutions like this.