I think that's a dangerous assumption. The hijackings are widely thought to have taken YEARS of planning. Bin Laden/Al Qaeda may have obtained nuclear weapons RECENTLY, but decided to allow the hijacking to proceed, since it had been 'in the pipeline' for so long. Remember, these hijackers (the leaders, anyway), probably knew a lot about the organization of Al Qaeda, and were therefore liabilities. By allowing the hijackings to proceed, not only did bin Laden show the world what he is capable of, but he eliminated the people who could break under duress or torture.
If he does have nuclear weapons, I look for him to use one SOMEWHERE soon, and then to say "See, I told you so. I have more, and you won't know where they are until I give the order to set them off." Think about it. The ultimate terror: A terrorist mastermind claims, then proves, he has a nuclear bomb. He then hints that he'll be setting more off, and that you can't stop it until it's too late.