Question: "In your opinion John, how likely or unlikely is the nuke scenario from the recent episode of "24"? -- ArbiterStrikesBack

Answer: It would be unlikely, but not impossible (given the damage a nuke would cause, that low probability certainly isn't comforting) at the moment because of the limited number of sources from which the terrorists currently have to acquire a nuclear bomb.

Incidentally, that's one of the scariest things about Iran getting nukes. It's not just that they might give nuclear weapons to terrorists, it's that Egypt, Syria, Saudi Arabia, Iraq, Kuwait, etc., etc., etc., would build them to make sure they could defend themselves from Iran. We could be in a situation, say a decade or two from now, where there might be a dozen places that terrorists could conceivably get nukes and the difficulty of figuring out who supplied the nuclear weapon after the fact could, in and of itself, make it much more likely that a nuclear bomb would be set off in an American city.

Moreover, keep in the mind that acquiring the materials for a dirty bomb would probably be very doable for groups like Hezbollah and Al-Qaeda. Imagine the panic, chaos, and havoc that would be caused by a massive explosion in the middle of LA that sent radioactive material for blocks in every direction. Not only would you have people killed by the initial explosion, you could have people getting sick from radiation, and a significant portion of a major city made uninhabitable for...who knows how long it would be?