The pro-reformist bloc which includes the former presidential candidate Hussayn al-Musawi and former president Mohammad Khatami, are also opposed to Ahmadinejad. The camp of the clerics is torn by internal power struggles and ideological discord. All that said, nominally, and to a great extent practically, the main power is in the hands of ayatollah Khamenei, who never misses a chance to demonstrate that. Besides, the mass protests in Tehran (interpreted by the regime as a direct attempt by the West to undermine their rule - no surprise there) have shown that the pro-liberal Iranians are also a formidable factor, although being suppressed with brute force - for now.
The US has imposed strict sanctions against Iran, and stopped buying Iranian oil - 60% of the Iranian economy is based on oil production. The EU, Japan and South Korea are also following suit. Moreover, they're tracking and blocking all possible trade relations and financial transactions of Tehran, which is threatening to remove Iran's ability to function properly as an economy. Granted, China and Russia are not part of the oil embargo, and China and India (plus Japan) are heavily relying on Iranian oil. Russia is also Iran's main supplier of military equipment, so don't expect them to do anything against Iran.
After the confusing signals about Tehran's intention to block the Straits of Hormuz and all access to the Persian Gulf, Iran backpedaled and removed the top general in the Revolutionary Guard who was in charge of the provocation maneuvers. In fact Iran would suffer the most from a possible closing of the straits, because that's Iran's main exportation hub. Of course, as one might expect the US didn't let this drama slide just like that and made if very clear that any attempts to block the Hormuz strait would mean an act of war, and the US would have to strike in retaliation.
At this background, Iran continues to develop its nuclear and missile programs, and their nuclear capabilities remain largely a mystery, but apparently Iran is getting closer to their goal. The assassination of the former nuclear physicist Mostafa Ahmad Roshan, probably done by western secret services, could only indirectly and temporarily slow down Iran, but it seems Iran has already passed the point of no return.
The Israeli prime minister Netanyahu came up with the hypothesis that the Iranian regime defies any logic of the nuclear deterrent and is theoretically ready for suicide. This hypothesis was supported by some of the GOP presidential candidates, Newt Gingrich comparing the Iranian regime to the rise of Hitler in the 30s (Godwin is all over the US political discourse these days!) While this is definitely a reach, the hypothesis about Iran defying the logic of the nuclear deterrent shouldn't be ruled out completely. In the worst-case scenario, Tehran might be prepared to risk even a massive strike from the US if that would be the price to pay for having its own nuke. After all, who could guarantee that such a strike would effectively remove the Iranian nuclear threat? Their facilities are deep underground.
That said, Iran is really a very weakened country economically (by the embargo) and politically (by its own extremist regime), and torn by internal discrepancies, and largely fallen into international isolation. Personally I don't think a preemptive military strike is even necessary in their case, be it from the US, and much less from Israel (Netanyahu may be all sorts of things, but I doubt he is suicidal). It seems Iran has been going toward an implosion for quite a while - one that would eliminate the medieval regime of the ayatollahs. So a skillful and timely nudge at the right place at the right moment would do a much better work than flexing military muscles - if Obama's new doctrine of "smart" foreign policy really counts for anything, Iran could be the best test for it. The alternative is going all G W Bush again, and sucking the US into yet another massive adventure with no known end - and in the middle of a financial crisis.
Surely, Iran should be handled with extra caution, while gradually increasing the pressure on the regime. And this pressure should be coordinated, and it should come from as many sides as possible. It's too easy to resort to the "last option" (which has been far from the "last" one in recent years, it's been more like the very first option if you ask me). Maybe hard problems require harder solutions that take a kind of effort that's different from the usual carpet-bombing. Although I'm sure there are plenty of people who'd love to see that happening again. I'm looking at you, CNN!