I guess we allow provocative runs on our ships until 400 yards out?
Depends on the situation. I'm pretty sure that if those on board the US ships felt threatened, that the Iranians would have been greeting allah in short order.
It all depends on the rules of engagement though.
Hell,
we've had Soviet ships ram US Navy ships during the Cold War.
But I doubt that the Iranians would be allowed to get too close. Obama may want to play election year politics and have loose rules of engagement.