I still think that Iranian threat is exagerrated. Uranium enrichment is long and boring job. Iran need at least 10 times more centrifuges than it has now to produce enough fission material for a nuclear reactor within the next couple of years.
Then it has to build a reactor capable of producing a weapon grade Plutonium. (The one which is being build cannot produce Plutonium even theoretically). All of this should be done in strict secrecy so that American satellites wouldn't spot it from the orbit.
Then Iran should create an intercontinental ballistic missile. Missiles it has now (Shihab 3) have the effective range about 1500 km - they still need several thousand miles to cover.
After all of the above is done (say 1 operating missile and 1 operating warhead), Iran leaders should all go crazy and launch it against USA.

And all of this is called 'a serious threat'?

I think it would be much easier to ship a nuclear device in a suitcase than to deliver it by missile.