A nuclear Iran means a nuked American city
A nuclear Iran would lead to a nuclear attack on the US because it makes sense according to realist and ideologically based assumptions. Let's say Iran gets nuclear weapons. Then, let's pretend Iran is not run by suicide bomber supporters, whose eschatology holds that the Mahdi will come only after the Muslim world is purified by fire and that the stated role of Ahmadenajhad is to bring the Mahdi. In other words we are running a scenario with a "rational Iran". If Iran gets nuclear weapons, Saudi Arabia and Egypt follow. Already these countries have responded to Iran's boasts, it "civilian" nuclear program, and open ICBM program with their own "civilian" nuclear programs. Saudi Arabia home of and exporter of the Salafist Wahhabi movement and the decadent Saudi clan is a spark away from revolution. Egypt is home of the Muslim Brotherhood, Egyptian Islamic Jihad (now part of Al Qaeda), and Gamaat Islamyia (the folks who assassinated Nasser). Hosni Mubarak is not healthy and the dictatorial regime only slightly more so.
An immediate result of Iran having nuclear weapons would be the retreat of the US from Afghanistan and Iraq. Our position would be untenable as Arab states would lose the will to go along with us. Al Qaeda and other terrorist groups would launch attacks through out the region to overthrow the regimes and there would be terrorism against American targets. (I'm not even touching Israel's situation here.) In a few years, emboldened by American retreat from the Near East and after a string of "conventional" terrorist attacks, nuclear war on America, Israel and Europe become rational. An attack though proxy is hard to trace and the regime in Tehran or an Islamist one in Cairo could easily decide that given their ability to respond, the destruction of Middle East oil facilities in a war, and the difficulty in proving culpability, the US may not respond with nuclear war.
This may sound far fetched, but is it any less rational than the belief of Japanese militarists that they could cripple our fleet in a surprise attack, and then conquer and secure most of the Pacific rim to such a degree that the US would enter peace negotiations in 1942? Is it less reasonable than Saddam Hussein's gamble that the US would not intervene in Kuwait? For 6 years, Iran has been arming our opponents in Iraq and Afghanistan. And we have sat idly by trying to negotiate. Why wouldn't they think us a paper tiger or hollowed empire ripe for destruction?
And this is assuming rational actors, not the Jihadists in Tehran. Assuming that the Islamists in Iran are serious, they've already stated their intent to destroy the US. The question isn't why a nuclear Iran would lead to a nuclear attack on America. The question is how long will it take and what will the consequences be?