they also are sensible enough in a scummy kind of way to use it as a bargaining chip, the iranians see the bomb primarily as way to convert Israel into a radioactive parking lot.
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.