A commentator at Matt Yglesias' site asks about those "serious foreign policy thinkers" who think a nuclear Iran could be a good thing. I don't know who's touting this line, or what they're thinking, but it sure is intriguing, and here's how I think this sort of argument could probably unfurl:

1) A nuclear Iran raises the stakes for Middle Eastern diplomacy and gives countries in the region more incentive to cooperate with each other, rather than going to war or fighting proxy battles through shadowy organizations. It also gives the U.S. and Europe more incentive to step in and mediate all these disputes peacefully. The obvious model here is Pakistan and India -- notice that those two crazy kids don't go to war with each other anymore. (Of course, if they did...)

2) Having nukes could cause Iran to act in a more stable and predictable manner, as it would no longer need worry about existential threats (from the U.S. especially) all the time. In particular, Tehran would probably stop running around linking up with terrorist groups and the like. Think about it: If you're Ayatollah Khameini, et al, and you're safe and sound with The Bomb under your arm, what possible incentive do you ever have to dole out aid and comfort to al Qaeda or Ansar al-Islam again -- especially when it may well come back to haunt you?

So those are the prongs. I don't think it's a flawless argument -- what happens, for instance, when it kicks off a regional arms race? -- but there you go. Being able to deal with a nuclear Iran on essentially realist, Cold War-esque terms might be an advantage for the U.S., given how difficult it is right now to figure out Tehran's often cryptic and complicated motives. Of course there are also the risks...