Noah Pollack being dense as usual is a good opportunity to repeat something:
Why is McCain allowing himself to be dragged into a debate about presidential-level diplomacy, when the more important question — and the question whose answer is more politically favorable to McCain — is whether diplomatic engagement will actually get anything accomplished? McCain should be asking Obama what concessions he realistically thinks he’s going to get from the Iranians upon going hat in hand to Tehran. UN Security Council sanctions have done virtually nothing to impede Iran, nor have EU diplomacy or IAEA reports. Russia and China continue to stand as the major impediments to the kind of UN sanctions that might so cripple Iran that it would give up its nuclear development.
The problem here is that, once again, we see hawks not understanding what diplomacy is. But think of diplomacy as a kind of bargaining. Like you might do at a yard sale or something. Diplomacy doesn't exist at one end of a spectrum of coercive measures -- we try war, we try sanctions, we try diplomacy -- any more than bargaining operates on a smooth continuum with robbery. The point of bargaining with a vendor is to see whether or not it's possible to find mutually acceptable terms that improve both parties' positions. In terms of diplomacy with Iran, the idea isn't that Obama's steely gaze would force concessions out of the Iranians, the idea is that we might be able to give Iran something Iran deems more valuable than weapons-grade nuclear material, and in exchange we would get verifiable disarmament.
The "something" here would presumably be some form of security assurances plus an accommodation to Iranian interests in Iraq, along with Teheran and Washington laying out a pathway to gradual normalization of relations in exchange for an end to Iranian support for terrorism and Palestinian rejectionist groups. Would it be possible to strike such a deal? Maybe, maybe not. But the purpose of a negotiating session would be to find out by attempting to do the bargaining rather than having five more years of back-and-forth blog posts speculating about the possibility. The general theory of diplomacy is that rational actors should, through negotiations, be able to achieve positive-sum settlements rather than negative-sum conflicts. It's always possible that your would-be negotiating partner will prove irrational (as George W. Bush did when he rejected Iranian peace overtures several years back) and the process will fail, but it's worth attempting in good faith.