I'm no expert in nuclear weapons, but what I've heard about them is that you can't just buy these suitcase nukes and keep them in the closet for years until you decide you want to use them. They have to be maintained like 2 or so times a year by someone with a significant amount of technological expertise. The longer he holds out on using them, the more likely they will become useless weapons. Someone correct me if I'm wrong.... I am not an expert either. I think that it is safe to assume that Bin Laden's organazation has acquired a variety of nuclear materials and perhaps complete weapons. But more recent articles suggest that he now has the technical expertise to construst, maintain, and detonate the devices.
It's possible. I'd like to see us come up with some sort of detection technology for these weapons, although I don't know if it is practical or feasable....