Michael Levi at TPMCafe says that Iranian security - assurances against attack - is a large part of the issue in the Iran nuclear standoff, but that it is neglected in recent proposals about the current Iran situation.
I would think this would be pretty obvious. Nukes are not for invasion purposes. They're for security, raising geopolitical standing, and national ego. Most assessments of the current issue focus on the offensive side of nuclear weapons, and mistakenly neglect the defensive side.
On one hand, there's good reason for this focus: Ahmadinejad's flaming rhetoric about Israel. We have those, especially in the US administration, who propagate the Ahmadinejad-is-crazy theme. By analogy, this echoes American rhetoric about Saddam Hussein in the leadup to the war in Iraq, and this of course presents cause for worry. We can't read the minds of the mullahs, but fairly good circumstantial evidence shows that Iran has played this entire game quite well. I also can't help thinking of the playground version: one way to counter bullies is to get that wild look in your eyes that says you mean business. Bullies, as cowards, will usually back off.
But on the other hand, the whole issue makes much more sense if we include the defensive side. Iran wants to be a serious power in the region. It already is, but nuclear weapons would consolidate this power like nothing else can. Iran would have to be included at negotiating and international policy-making tables. That's a large step up in international standing from the one the country has lived through during the past few decades - of being labeled a "pariah state," part of the "Axis of Evil," of living through economic sanctions and political snubs. Scanning the political landscape, Iran would have to assume that nuclear weapons would change this position. At the very least, it would be in the position of North Korea - not terribly involved in international affairs, but the dangerous state that you leave alone. North Korea, however, is exceptional, and I doubt Iran - which does have good relations with many countries, and is itself a diverse and well-educated society - would ever be in a similar position as an isolated little universe of its own.
In the meantime, Iran does have its own low-tech deterrence policy, noted below: an estimated 40,000 trained suicide bombers ready to deploy around the world. The US is caught in the position of being between the rock of the "war on terrorism" and the hard place of Iran as a nuclear state.