As has been pointed out elsewhere, there are literally thousands of disused airframes nestled in various out of the way corners of the globe. If your goal is really to fly an explosive laden jet into something, absconding with a loaded passenger jet is about the least clandestine method imaginable.
And if you decide to go down the route of hijacking a passenger jet, why not hijack one near your intended target?
Chances are there are hundreds of aircraft flying right next to your target everyday, simply hijack one of those on the day and fly it right into your target. 9/11 already proved the viability of this tactic.
