Can't Iran exit the NPT if it provides enough evidence that it needs them becuase there is a possible nuclear threat to it from another nation (namely Israel)? Everyone has to realize is that there is no definitive proof that Iran is seeking nuclear weapons. We also have to ask ourselves whether the US's excessive use of force in the region (and world-wide) drove Iran to think that it needs these weapons for its survival (sort of how Israel thinks it needs such weapons for it's survival).
Honestly, IMO, the quickest way to nuke-free Middle East is if Israel gives up it's own nuclear weapons.
Any one can defect from an international agreement, including the NPT. In fact the North Koreans just did that - they ractified NPT initially than opted out & earlier this year declared they tested a device.
Iran can also do the same but it are the events/situation that will follow, after opting out of the agreement, which are more dangerous perhaps - especially in case of Iran.
1. It would prove that Iran is seeking nuclear weapons & in such case both Russia & China probably wont be able to do much to save Iran from any possible wester military action.
2. China & Russia might also have to stop military aid/sells to Iran under such conditions.
3. Big package of economic sanctions. Americans would probably make sure that Iran friendly countries cut off all economic ties with Iran (probably an end to IP-I gas pipe-line which seems to be a major economic project in the region at this moment).
4. Other neighbouring countries such as Saudi Arabia may attempt to proliferate & Israel probably will be inch closer to testing its own devices.
This means complete isolation of Iran. For this reason we see Iran stating its program is for peaceful purpose - something NPT allows & that is how Iran has to keep it. They can & probably are using the "peace full purpose" as an Umbrella to cover their actual intentions but this umbrella can never be removed however there are always risk of it being torned apart.
As oppose to what "Chrom" just said above, I think Nuclear Iran would destablize the "greater region". The Indo-Pak nuclear weapons initially did very little to prevent war - in fact the countries were at the brinks of war in 2001-2002. It was the international pressure on the both countries which averted the war. Both Pakistan & India had to face economic loss (Pakistan's economy had started to improve while India's well established but 2001-2002 standoff vertually reduced both countries economy to nill).
In case of Iran & Israel;
1. Iran is very unpredictable & Israel pursues very aggressive defence policy.
2. Both countries have no diplomatic ties, unlike Pakistan & India, so there is greater chance of misunderstandings.
3. International pressure may apply to Israel but Iran might reject them (including pressure from China & Russia).
4. Iran having very little conventional strength has greater chance of using nuclear weapons as means of war. In this case Israel having no other option might also have to employ nuclear weapons. "MAD" in action.
I do see greater drastic scenario. But till date we have seen credible deterrence coming to exitance than a war - but that has been because of more social-scientific approach than emotional. In case of Iran-Israel there is very little social-scientific approach & more emotional approach.