If you back the opposition they are 'tools of the west' (which they usually get accused of anyway, but now there would be real proof) and if you don't you leave a nuclear country in the hands of a gang of nutbars.

However, as has been pointed out, North Korea is a nuclear country in the hands of nutbars and isn't actually all that scary. A nuclear Iran would probably be similar - the only problem would be if they decided to launch a military action that they figured no one would counter because of their nukes. I can't see that as likely as if the military were that occupied the resistance might just stage a coup d'etats while they were off fighting.

The scariest thing is Ahmadinejad, who is a true loon, but I'm not sure his handlers would really let him near the button.

The second scariest thing is Israel. They have, in the past, been known to pull strikes in other countries that they feel have threatened them and Iran's nutjob president has done just that. Contrary to anyone's belief, the Israeli Air Force could deal very well with any launching sites and if Iran were to start handing off nukes to Hamas or Hezbollah, well, that would not end well for Iran.

It's a thorny question. A lot of the Iranian PEOPLE are pretty forward thinking - Shiite Islam has often been less backward than Sunni in a lot of ways - but their government is run by a nutjob with some very scary back room handlers who are both pretty proficient while still being pretty out of touch and also are well supported by a significant and armed portion of their own population.

Sanctions, as always, are largely a waste of time. They don't really much discomfit the senior people and give them a propaganda weapon to use against their enemies. A war isn't really the answer.

It is interesting to imagine just what Iran might say if, however, all their research and missile facilities were blown to bits mysteriously in a single night without anyone taking credit for it...