The way things went down on Sep 11th, won't happen again for the simple reason that there has been a sea change in the psyche of the public, and the passengers would almost certainly take matters into their own hands. If you think about it, an essential ingredient for the success of the hijackings was that 200-odd passengers sat there like lambs to the slaughter while these hijackers, armed at the end of the day only with small knives, took them all to their deaths.
But if you put yourself in a pax seat tomorrow, and the little arabian bloke next to you jumped up waving a Stanley and said he was taking over the aircraft, what would you do, sit there fat dumb and happy? I reckon they'd be torn limb from limb before they even got two rows towards the flight deck.
Of course we're still at the mercy of bombers to a certain extent but to be fair, the Bali bomb showed us that it's just as easy to get blown up in any public place as in an airliner. It isn't perfect and never will be but but at least we have some attempt at security measures in place in our industry, the shopping malls and railway stations and millions of other public congregation places do not. In a lot of ways I'd rather be sitting up the front of an airliner these days than in some other soft civilian target.