On an NBC (Nuclear, Biological, Chemical) warfare course about 9 years ago we were told that during the cold war the Soviet Union developed a nuclear device that could fit in a briefcase, these were sent to agents in western countries to be used in the event of a war between the Warsaw Pact and Nato in places like a locker at a train/tube station in London, near the City. The explosion would devastate an area about one square mile and cripple the countries economy.
130 of these were manufactured, and at the end of the cold war they were recalled by the KGB, however only 113 were returned, which leaves a discrepancy of 17 which are out there somewhere. It's thought that the KGB kept these 'just in case' they were needed.
Now I don't want to frighten you guys, but it's not beyond the realms of possibility that a renegade Russian agent could sell one to an Al Qaeda operative is it seeing as Bin Liner is a rich rich arab?