"IMHO that's why the only viable last-resort military option against Iran will have to be a bombing of the alleged nuclear production sites by the U.S. (NATO?)"
You are correct, however even this is extremely complex and not at all appealing for several reasons.
First of all, the Iranians are smart. They watched (in 1981, I think it was) as the Israeli Air Force took out Saddam's Osirak nuclear plant in Tuwaitah. The attack worked splendidly - the plant was more or less above ground and was not really hardened (I think it was still under construction), so the Israeli attack completely neutralized the plant - and Iraq's hopes of constructing nuclear apparati.
Watching this, the Iranians have gone to extreme steps to prevent this from happening. They have built multiple facilities, all of which are hardened, and the majority of which are below ground. In short, cruise missiles and bunker busters are not sufficient enough to make sure the plants and all the nuclear material has been destroyed.
This means that any airstrike would have to be a massive one and a sustained large-scale airstrike requires 1) complete air superiority - no defensive fighters in the sky, no SAMs or radar posts 2) air-to-air refueling and 3) enormous pay load packages. Number 1 alone is enough to significantly halt any plans - it took several thousand special forces, several hundred assault helicopters and jets, and several weeks to take out Iraq's air defense network in the first Gulf War - which is the closest approximation to what we'd be up against in Iran.
And so clearly everything is dependent on the US. No other country has the military hardware and variety of arsenal to accomplish this mission. Plus, the United States has one hugely significant tool that can, in part, negate a lot of these difficulties: the B2 stealth bomber. If we trust their stealthiness, and we are prepared to lose several - no small if's - it is conceivable that a concerted surprise campaign of B2's could signifcantly damage the targets. But again nothing is guaranteed. And also, if we lose a couple of those B2's, in addition to the lives of their crewmen, we've also lost a lot of secrecy to reverse engineering processes of foreign nations. I'm not sure how much of a concern this is, but I'd imagine it's notable.
Now for the bad news. Iran is a HUGE country. That means we'd probably have to refuel in flight, which complicates things even if its over friendly airspace - Iraq or Afg. KC-10s are hardly stealthy and almost any Iranian fighter pilot could conceivably trail one to a B2 - since the airdefense networks of Iraq and Afg are non-existent mostly: actually I take that back, I bet we'd love for an Iranian fighter to stray over Iraq.
More bad news. 135,000 of the US' best soldiers live right across the road in camps the size of small cities. It is not hard for the Iranians to launch several hundred medium range ballistic missiles back at us and kill up to 30,000 of our guys, not to mention Brits and the other nations still over there. Once that's over, then the weakened force would have to go out to on fire Shi'a neighborhoods where we've been drawing DOWN troops or adding Iraqis (same difference) and that have heretofor, exception an Najaf, stayed quiet. Moqtada al-Sadr has already said he will rise up in defense of Iran were it to be attacked. Al-Zarqawi who himself ran an AQ training camp in Herat, Afg near Iran in the 80s would be beside himself. He could attack the Iraqi Shi'a who would at this point attack back (their towns are in chaos anyway), thus drawing the country into a civil war - one which the US would perversely need to help the Sunni guerilla fighters

to prevent Iran from dominating more real-estate. Yikes.
And you'll like this: our arsenal of weapons to attack hardened underground targets is limited. We've got a few different types of massive bunker busters - only a few of which could be dropped from the finite number of B2s - and that wouldn't guarantee beyond 70 or 80 percent at most complete destruction. All that adds up to is nuclear missiles being the most comprehensive option. I'd imagine that this is in no way currently ruled out. Yikes again. But then you've got an eighth or so of the world's oil not only offline, but radioactive. As well as a radioactive US NAVY and possibly a radioactive southern Iraq - depending on a couple factors.
And none of this is considering Iranian asymmetrical counterattacks, Hezbollah, Islamic Jihad, etc, which are equally unsettling.
Yet staring us in the face is the option of Israel launching dozens of nuclear missiles over our 135,000 soldiers and equipment towards Iran, still with the above setbacks.
All in all, the options are quite unappealing. Good luck, DoD, you've got your hands full with this one. As Senator McCain said a few weeks ago, that, barring diplomatic efforts, war with Iran is the best of a list of terrible options.
IMHO, the best solution would've been to snatch Senor Ahmadenijad when he visited the UN here in New York half a year ago. Of course, the UN would've officially disintegrated at the point so - wait, maybe two birds for the price of one with that one.
...Except for WFP, UNDP, and one or two others - I like them.