I think you are putting two separate issues together. Limiting strategic nuclear warheads will not effect anything if Iran has a missile capable of reaching the United States.
If you believe that MAD works, then an Iranian weapon does not matter. (You also have to add in time to allow for Iran to be able to put a warhead on that potential missile)
Additionally, if you do not believe in MAD, the current missile defense structure in place (GMD sites in CA and AK) will offer at least a limited protection of the US mainland.
The problem will be if they would use such a missile against Europe or other sites, but the time it would take them to build multiple missiles, and multiple warheads, plus develop the ability to place a nuclear warhead on a missile, will most likely take a few years, and by 2018, (at least in theory) the next stage of missile defense deployment will have occurred, and will offer a much better shield of Europe.
Certainly an Iran with long range missiles and nuclear weapons is not a positive scenario, but I think this story by itself does not add anything to the actual threat we might face from them.