As for nukes exploding over US city or several… At the moment, ballistic missile attack is not likely (NKors may be able to reach SanFran, though). Suitcase nuke is a different matter, but the delivery is somewhat challenging and those that originate from late 80′s are duds already. Radiological bomb is more likely.

BTW, Russkie are considering sanctions against Iran. Medvedev said that if Iran does not accept the conditions of western countries, they’ll be forced to sign on US sponsored sanctions bill.