Sitting here under our first ABM umbrella, I would be inclined to say we can afford to let them shoot their wad, so to speak. It is nukes not terrorists. Different game, and we have a lot of experience with the nuke game.
*sigh*
If Iran hits us with a nuke, it won't be fired, it will be smuggled in, possibly across the Mexican or Canadian borders. Our ABM systems will be useless.