On Iran the US has fucked around far too much. Here is a country that has consistently supported your enemies abroad, sought to destabilize Iraq, made promises of genocide on numerous occasions and now has a true believer in charge who drops diplomatic nicities. Those nuclear facilities, however distributed, should be wiped off the face of the planet - diplomacy will not work, the regime has made it abundantly clear where it stands. Waiting for them to get weapons and putting sanctions on the country would be disasterous, simply allowing the weapons to be developed would be disasterous. We have all the bad choices as a result of convergence of events.
The possibility that the man is a true believer does not seem to even be entertained, we will see how he "panders to his base" when the weapons are actually there, 10 to 100 million dead in the Middle East could be our oft-repeated "never again" where the world can go through the motions of pretending it couldn't have known or that it was all bad.
"The smoking gun that could come in the form of a mushroom cloud." - Condoleeza Rice - how about some fucking consistency
(not invade Iran, there is no need - remove the chance of it going nuclear and start supporting internal dissent a lot more. US troops in the country will make the current slightly pro-US and fiercly anti-Mullah attitude shift).
If I may also highlight a few key differences between Iran and Iraq. In Iraq the USA managed to loose a lot of favour by not only abandoning the Shiites to Saddam after the Gulf War but also letting Helicopter Gunships crush the rebellion - this was done in the name of stability because the realists felt that having a Shiite government would be giving too much influence by Iran - fifteen years later and now there will be a government of national unity but with fair Shiite representation. In Iran the USA hasn't made or broken promises that resulted in mass death of the people.
The numbers of people being killed is different. Iran is a softer dictatorship than Iraq, the grip on free expression does not rest with the Mullahs. People are not dying in the tens or hundreds of thousands at the hands of their government. More people would die if the regime was removed with military force than if it is left in place as it is. The calculus here could change if sanctions were applied or if the regime gains nuclear weapons and has free hand to crack down on internal dissent without fear of international reprisal (the nuclear umbrella).
In terms of internal opposition Iran has a much better base with which to forment revolution against the Mullahs. There remains room for less overt means of excercising power to a desired end. This had been attempted against Saddam and had failed spectacularly.
Iran is a very different situation than Iraq, all out war would not deliver any desirable ends, allowing nuclear weapons in the hands of the Mullahs is not desirable. A limited strike against nuclear facilities like the IAF one against Osirak followed by more support for the Iranian pro-Democracy movement is the most favourable that results in the least death.