9/11 has changed us. Now we know that they intend to die in their acts of terror, so there is no reason to "obey" them. Previously, people fell in line with the terrorists because they only wanted to divert planes, not explode them with all aboard.
To get control of a plane would be very difficult now, IMHO, but perhaps blowing one up would not. Perhaps all that is left to the terrorists is to blow up a plane from within when over a city. I can't see how they would get control of passenger planes and actually get to fly them into buildings. I repeat that I think it cannot be done today.