Not only is Khamenei a qualified arbiter of Shi'i shari'a, he will also be the complete arbiter on US-Iranian bilateral relations. Roger Cohen's piece in the NYTimes today hits the issue right on the head (although I may be a little more skeptic of what can turn Khamenei towards the US):
I’d say the central Iranian political phenomenon of recent years has been the reinforcement of Khamenei. How to engage with Iran begins and ends with him.
While rhetoric is flying from all talking heads of the Islamic Republic of Iran (IRI), Ahmadinejad, Foreign Minister Mottaki, Majlis speaker Larijani, Rafsanjani, etc., etc., it is Khamenei and the hierocracy that will have the say. While the "elected" President and his cadre of cabinet ministers, along with the legislature, may have a more tempered tone vis-à-vis the US, the line from the hierocracy; its mouthpiece Kayhan News; and its military wing, The Islamic Revolutionary Guard Corp (IRGC), is clear: There will be no rapprochement from the IRI at this moment, and the government can talk all it wants about it, but the Supreme Leader has total control.
The “principalists” from the hierocracy take a harsh, and highly distrustful tone. Mullahs from Khamenei, to influential Guardian Council and Assembly of Experts members like Ayatollahs Jannati or Khatami (not the reformist ex-president hojjat-e Islam Khatami) have been using ‘Revolutionary’ rhetoric as of late, displaying the foreign distrust so crucial to the ideology of the IRI. But it can be noted that this rhetoric is not completely antagonistic, such as violent slogans or demands for US “death”, towards the US. This is particularly noticeable in the official Tehran Friday prayers conducted over the last few weeks by Rafsanjani, and Ayatollah's Khatami and Jannati.
But even with forthcoming overtures of diplomatic relations from the Obama Administration along with tempered mullah rhetoric, Khamenei has shown a very important characteristic of his worldview: his complete, intense, and full-scale mistrust of US interest. He characterizes the US as “feeling weak”, enlightening his belief that US overtures are only based on its now weakened regional position opposite Iran. As is laid out in Article 110 of the IRI Constitution, his power over policy is absolute with his ability to say 'yay' or 'nay' on any policy enacted by any body in the IRI government structure.
While some in the government have displayed a desire to engage in the possibility of bilateral relations resuming, the Supreme Leader expresses his deep-seated and personal mistrust of foreign approach (remember he was arrested and tortured by SAVAK, believed to be an arm of the CIA, six times before Khomeini's return). From his rhetoric there is no reason to believe he will reign in all discourse of possible engagement by his government, but Khamenei’s position will unlikely change in the current climate, especially as the stakes raise with uranium enrichment ongoing.
Am I completely cynical, desiring military force, or a drop in diplomatic tone by the Obama Administration? Do I think engagement can only actually happen after Khamenei dies (probably a long time from now)? No, definitely not. But I don't see elections in June actually bettering the possibility, whether Khatami (or Mousavi or Rowhani, if they run) wins against the "principle-ists" or not. The US must engage Khamenei (preferably through back channels to begin) and will have to accept nuclear technology existing on IRI soil. Khamenei has already issued a fatwa against nuclear weapons so he can easily prevent weaponization without issue, but nuclear technology in general carries the torch of cultural and scientific self-sufficiency, both major principals of the Revolution that Khamenei is vehemently loyal to.