I'm sure there will be folks who drop into the thread to tell you how silly this whole nuke thing is--and of course, they're right: why, sneaking a nuke into this country and detonating it is every bit as outlandish a notion as flying planes into buildings.
Unless al-Qaeda has a Cray or a Blue Gene/L for simulation and testing, they would have to actually test a nuke, or overbuild and risk detection.
It is possible to spread nuclear material around, but that would be very localized, even if hidden in a conventional bomb.
But, you're free to inflate al-Qaeda's capabilities when the best they've been able to manage in the last year is to blow up a few subway cars in the London Underground.