Uh, we've been down this road before. Last time Israel bombed it. What should the US do about it? Tough call. Each choice has consequences. The US has not been opposed to the reactor, only the enrichment. So obviously the US can't throw a hissy now. I think the US should demand rights to inspect the operation but Iran claims its being monitored by the Ewe In. Once they start operating the reactor (and enriching Uranium) it will be too late, short of an all out attack. I don't see that happening.
What do think the chances are that Iran develops weapons grade Uranium or Plutonium?