Actually, I think the higher probability of occurance is the creation of smaller, portable weapons (including dirty bombs) to be used in plausible deniable terrorist attacks. When DC blows up and you can't trace the source of the bomb, think Iran. Expect them to use processes to make the nuclear materials to appear to be from another source (based on trace elements). The mullahs know how our politicians work and will use the media against us in this scenario. Iran will deny giving the terrorists the bomb, our media will hamstring the politicians since there will be no "smoking gun" linking the bomb to Iran, and given the wrong administration, we could destroy our Constitution in the process.
"Iran will deny giving the terrorists the bomb, our media will hamstring the politicians since there will be no "smoking gun" linking the bomb to Iran"
You mean like this?
"Tariq Aziz, a senior henchman, then chipped in with some supportive thoughts of his own.
"Sir, the biological is very easy to make. It's so simple that any biologist can make a bottle of germs and drop it into a water tower and kill 100,000. This is not done by a state. No need to accuse a state. An individual can do it."
http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/02/17/wirq117.xml&sSheet=/news/2006/02/17/ixnewstop.html