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.