If you think through the consequences of attacking Iran preemptively, the risk-reward ratio seems most unfavorable.
The best-case upside is that Iran is disarmed permanently, and everyone shrugs their shoulders and accepts it. What are the chances, really, that will happen?
The worst-case downside is appallingly bad. It could rally the Iranian population behind that mullahs - a population that right now is very unhappy with their leadership, and somewhat pro-American. That would all be swept away, and even worse, it could rally the majority Sunni muslims behind the minority Shi'ites in Iran in a way no other event possibly could. It would be the polarization of the West versus Islam that Osama bin Laden has long been seeking. It could end the disunity in the Muslim world that thus far has worked to the West's advantage.
'Anti-zionism' is the one tie that binds Muslim states that otherwise would be enemies. Israel does not need to participate in a preemptive strike for it to be the target of Muslim ire around the world - the assumption will be that the U.S. is merely doing the zionists' bidding. Reason enough to attack Israel, in their view. If you are concerned about Israel's security, think of the chances that Iran might attack Israel preemptively. It may or may not happen. If Iran is attacked, however, it becomes a certainty, with the rest of the Middle East quite possibly joining in. Lebanon, too, is put at risk as Hezbollah becomes activated in the conflict.
It would make things in Iraq terribly difficult. Iran has been a very bad actor towards Iraq, this is an understatement. It is nothing, however, compared to the resources and manpower they would throw at Iraq in the event of the outbreak of war. Think of the damage they have already caused in a clandestine way, and now multiply that, and add overt military operations versus Iraq on top of it. Afghanistan is likely to be even more vulnerable.
Iran has the support, money, and safe haven to prepare terrorist strikes against the West that al-Qaeda now lacks. Iran would certainly call on what remains of al-Qaeda to help in its terror war against us. There are already operational ties between al-Qaeda and Iran. War with Iran would certainly result in an outright alliance.
The mullahs are not that bright, they woefully underestimate the strength of the U.S., and they are bound to eventually offer a reason to attack that even the Europeans and much of the Islamic world will support. We cannot wage war against Iran successfully until they are completely isolated, and they are not at that point now. Also keep in mind that Iran's biggest threat is an internal one. Iran is prone to labor and student strikes, protests from ethnic minorities (such as the Azeris), opposition parties and even terrorist groups operating against the regime. It is actually quite amazing that the mullahs have not yet been toppled.
Attacking them may in fact be the only thing guaranteed to save them.