I still think all this focus on airplane security is a show. Security on trains, ferries, malls, stadiums, and arenas are still pretty lax.
And don't forget that fellow passengers stopped the underwear bomber even when security failed. If it's just one fool sneaking a knife or gun on board, as long as the cockpit is locked, it shouldn't be a problem if he succeeds. When it comes to chemical stuff, it's definitely trickier, but I don't think an x-ray or metal detector would get everything anyway.
And again, if the cockpit is locked, they can't hijack the plane anyway so all they'd kill are the people in the plane at most. If the terrorists were smart, they'd start going after sold-out stadiums or somewhere else where they'd be able to kill a lot more people.