Via Andrew, Jeffrey Goldberg contemplates the problems an Iranian bomb might create: 

Something else changes: Terrorist groups that threaten, or have threatened, American targets – terrorists in Iraq, Hezbollah in Lebanon – will come under the protection of the Iranian nuclear umbrella. Hezbollah’s rockets have helped the group establish a local deterrent to Israeli attack; an Iranian bomb would strengthen Hezbollah in Lebanon, and well beyond Lebanon. 
An Iranian bomb would also set off new tension between India and Pakistan, an ally of Saudi Arabia that would almost certainly turn to Pakistan for help with its program, making the Indians, who are already distressingly close to India [sic], exceedingly nervous.

I assume that he means to refer to the relations between Iran and India in this last sentence, since India has been building the same sort of strategic relationship with Iran that it is now cultivating with Afghanistan in an effort to check Pakistan on two sides.  Including Pakistan and India in the discussion is important, because it is not possible to understand an Iranian pursuit of nuclear weapons without bearing in mind its long-running rivalry with Pakistan for influence in Afghanistan and in the region generally and the perceived need to match Pakistan’s nuclear arsenal with one of its own.  An Iranian bomb would complicate the situation and could encourage more proliferation, but one can also see how it could make an Indo-Pak nuclear exchange even less likely.  If two of Pakistan’s neighbors have a nuclear deterrent, the consequences of using its weapons become even more disastrous and so, one assumes, less likely.  

While it would be ideal if no states possessed nuclear weapons and the increase in the size of the nuclear club makes total nuclear disarmament even more difficult, there is some reason to believe that as more states acquire nuclear weapons there will be fewer occasions for large international conflicts because of the severe dangers that would come from escalating any disputes into a full-scale war.  The Kargil war in ’99, in which Pakistan’s military tested the limits of how much aggression its newfound arsenal would permit, could have escalated into a larger conflict, but it was in some significant part because both states had nuclear weapons that ultimately neither expanded the conflict.  As more states acquire these weapons, which would seem to be a more or less unavoidable consequence of globalization and the spread of technologies, low-intensity and proxy conflicts will probably become the norm even more than they already have done.  If we think of them as attempts to thwart the spread of knowledge and technology that is part and parcel of the global economy, wars for nonproliferation are a bit like fighting against the waves.

Meanwhile, the last time Hizbullah really threatened American targets was in the ’80s, so even when we are trying to discuss the consequences of an Iranian bomb beyond Israel’s security concerns we keep coming back to talking about threats to Israel.

Earlier, Goldberg asks:

Can we really live with a Middle East that has eight or ten nuclear powers?

It seems very improbable that there would be that many.  It is not difficult to imagine an Iranian bomb prompting Egypt, Saudi Arabia and Turkey to try to acquire a bomb, and Syria might follow suit, but I don’t see how we get to eight or ten states.  All of those, together with Israel, would still make for six new nuclear states, of which the Saudis are probably the most worrisome.

At the same time, instead of discouraging Afghan-Indian ties, as Kaplan recommends, the U.S. needs exactly this kind of strategic balancing to succeed to make clear to Pakistan’s military establishment that Afghanistan is not the ISI’s playground any longer.  India has a permanent, vested interest in cultivating a strong, stable Afghanistan; our interests here, while important, are transitory and will end in the near future.  Our presence there will eventually end, so in the interests of regional stability Afghanistan needs to have powerful regional allies that can counter the most dangerous elements within Pakistan’s military.