I think Iran has a nuke. Most likely from Russia...purchased for hard dollars.
The Iranians and Russians have close ties since trying to control some of the Republics that border Iran and have oil interests.
What Iran wants is the ability to manufacture nukes and to have a superior force able to face anyone in the region. Since Israel is rumored to have over 100 they have a way to go. They are in the minority after all, in an area where being in the minority gets you "ethnically cleansed".
Obviously since they are a state sponsor of terrorism the more rational governments of the world will not allow that to happen. Either us or the Israeli's will take out the capability to mass produce nukes soon. I think after we have perfected a bunker buster nuclear device...which should be soon.
I don't think they will use one against us since they have to much to lose. Cities etc. make easy targets and we can hit theirs with ease. That tends to inspire restraint.
I don't advocate nuclear war, first strike or anything of the sort. I simply point out that the realty of the world as it is now and the utter stupidity of trying to use one against the US...and expecting to survive afterwards.