Bush said that O.B.L. was attempting to acquire nuclear material, which is a given since some people have already been arrested in the attempt. That's a ways away from actually have done so. The timing of this, coming as it does with some bad reversals for the Taliban, are no mere coincidence, in my opinion. O.B.L. wants to freak us out, and declaring that he has nukes seems like a way he'd try to do that. Deploying a nuclear weapon seems like something he'd have done at the first. If he has one and saved it until now when we're on hyper-alert and hunting him like the dog he is, he's not much of a tactician.