The United States will NOT attack Iran.
It will not do so for several reasons.
The backlash from Shiites in Iraq would be huge. If the US thinks it has a hard time holding together support for its continued presence in Iraq now, it would become impossible after an attack on Iran. The Shiite clerics in Iraq would send the people into violent resistance, that is if they would need any encouragement in the first place. The US might as well shoot itself in the head as far as creating any kind of lasting pro-US regime in Iraq.
Iran is actually an ally for the US in its struggle against Saudi fundamentalist Waahabist Islam. As predicted by Juan Cole, Saudi Arabia will use its huge oil profit windfall to further strenghten itself and its worldwide Wahaabist message, which is anti-USA at its very core. Iran's Shi'a leadership and the Persian people are natural and historic allies against Saudi Wahaabism.
Militarily, an attack on Iran would bring ruin to US relations with China and Russia, both of whom have been working closely with the Iranian regime, in many areas of military and economic cooperation. An American attack would further isolate the US and would give rise to more anti-US regimes along the lines of Chavez' Venezuela. Can you imagine dozens of Chavez-like leaders biting at George Bush's ankles and causing him to sweat profusely everywhere he goes?
Iran's potential nuclear weapons capability actually represents a counter-weight to other Sunni Islamic nuclear powers and potential nuclear powers. Iran is not so much a threat to the US as it is to Saudi Arabia, Pakistan, Syria, and other hardline Sunni-dominated countries. An attack on Iran would destroy this balance and give hope to anti-US Sunni rejectionists in Iraq, not to mention anti-American Wahaabists in Saudi Arabia.
The alternative to a US attack on Iran's nuclear weapons potential would be an Israeli attack, both unthinkable for Israel and impermissible for the US. Despite Israeli saber-rattling, as well as US threats, the attack will not happen.
If I am wrong, and I grant my fallibility, it will only be because George Bush is even more foolish and reckless than I think he is.