Gentleman Aviator is right. The only way to block access to the flight deck is blocking it out compleatly regardless of threats or violence in the back. I think airport security is an other thing; those determined enough can and are able to get their will done. Stupidity and plain evil can go a long way. On the other hand we could have armed guards on every flight.
Flying is about compromise. It can be dangerous but we still fly. The security measures taken and inflicted in the future will be a compromise of price and comfort. It's safest to bombserch every luggage, but it takes time and money and therefore not done - yet. I belive that airtravel as we know it will forever change. God willing, security measures will filter out the worst, ie. the un-luckiest, but those crazy enough to do this kind of terror are still living and even contemplating something similar. The only thing that can really stop these people is morality and respect for life.