A nuclear Iran is not in the long-term best interests of the United States, of course, but we have short-term interests, as well, and they conflict with what some see as Israel's interest. Second, I've moved to the belief that the Iranian government is not so much a messianic apocalyptic cult, as Netanyahu described it to me, but an oppressive military regime with a superficially Shi'a agenda. Its real agenda, it seems, is self-preservation, and people interested in staying alive, as individuals or as a collective, don't launch nuclear-armed missiles at a nuclear state with a second-strike capability.
The Iranians understand that Israel could obliterate Persian civilization. There are some mystics -- Ahmadinejad, for one -- who might want to carry out a seemingly-irrational attack on Israel for their own millenarian reasons, but my impression, to date, is that other Iranian leaders would rather stay alive, and these men have a great deal of sway over the nuclear program.
As I've written before, I don't discount the long-term dangers of a nuclear-armed Iran. A nuclear program will help Iran achieve hegemony in the Muslim Middle East, and the gravitational pull of a such a powerful Iran will do great harm to the peace process, such as it is. And I obviously think that this is the most serious issue facing Israel, and one of the two or three most serious issues facing the U.S., today. But so far at least, no one has convinced me that an armed attack on Iran's facilities by Israel would a) work, and b) make the world a safer place and c) protect the Jewish people from a second Holocaust.