A detente is among us--at least, according to the rhetoric of the Iranian and American leadership. After all the hype, the dozens of news reports scremaing that a Bush attack was imminent, missile tests, Iranian flyovers, Syrian defection, UN sanctions, everything--the two sides are finally ready to hammer out a deal. The deal's not there, but the intent signals a true detente.
First: The Americans have offered to be a part of nuclear negotiations with Iran, a policy reversal that has probably come with some quiet knowledge that Iran is willing to barter with its nuclear program, rather than keep it against all costs. Such a change in policy came when the North Korean regime made the same quiet decision, and has led to the de-Axisification of Kim Jong Il. More importantly, the Iranians have agreed to the deal, and claim to be looking forward to the talks.
Second: The US is now very likely to get an interests section in Tehran--and an Iranian one would likely be placed in the US, later. Not an embassy, and not any sort of rapproachment. But, it's an open overture by both countries to their own people and the people of their adversaries that they are willing to talk, on a relatively equal level.
This is not a random policy-change by Bush. Bush sticks to his guns, and does not change his mind. He does not believe his previous policy was wrong, but that it worked. How it worked is unclear. It is likely that the Iranians either A) were willing to give a great deal on Iraq or B) quietly admitted that they could trade away their domestic nuclear program for something tempting--like Western assistance in building protected nuclear power plants (with tamper-proof centrifuges), to reach a middle-ground on the issue that has been glaringly obvious for years.
If Iran did make a sudden shift in strategy, there is a reason as well. Unlike Bush, Ahmedinejad has to worry about public opinion, and in particular about the religious leadership of his country turning on him (and essentially stripping him of his power). He's got to keep more people happy than Bush, and it's possible the psywar of the last few years, the Syrian departure, and security successes in Iraq have shaken Iranian confidence in the "brinksmanship" strategy just enough that they are going to deal, whether Ahmedinijad and Bush want to or not.