You are correct that the U.S. would be hard-pressed to deny green-lighting an Israeli attack on Iran if Israeli planes fly over Iraq. This would not, however, be likely to drag the U.S. into war with Iran. It’s hard to imagine Iran doing something as counter-productive as reacting to an Israeli attack by declaring war on the U.S., seeing as we now have a pretty large concentration of troops right next door to them that already has experience in implementing regime change. Instead, the Iranians would probably take revenge against the U.S. by stepping up their funding of anti-U.S. elements among Iraqi Shiites.
The diplomatic attempts to impede the Iranians’ nuclear program are starting to smack of Chamberlainian myopia. The Iranians are not even bothering to disguise their position that they have an "inalienable right" to produce nuclear fuel, and as Ahmadinejad has declared, even share their technology with other Muslim states. The U.N. is useless here since Russia, which is making big money helping Iran to develop its nuclear program, has a veto in the Security Council. So the choice that confronts us is quite clear — either the Israelis take out the program militarily, or Iran will become a nuclear power.