Dan Drezner has a very good post on the problems of credibility in the stuff being discussed about Syria these days. The focus has been entirely on the credibility of the US following through on its threats, but these discussions, as Dan rightly notes, credible threats must not just be credible about using force if the other side does not give in but also NOT using force if the other side does concede.
This is the Big Brother problem: that your big brother may promise to stop hitting you if you give him what he wants, but he just keeps hitting you since... he is your big brother.
One of the key problems of the axis of evil regime change obsession since around 1998 (when folks pressured Clinton to make regime change the US objective in Iraq) is that it makes little sense to bargain with the US if it is going to keep trying to overthrow you not matter what you do. Forget Iraq, how about Libya? Gaddafi gives up his stuff and tries to turn a new leaf (hanging out with big names in political science to prove his street cred!?), and the US seems to go along with it. Until Gaddafi hits a bump in the road--the protesters he is seeking to squelch. Then the US policy of preventing the loss of lives turns into regime change (for not entirely bad reasons since G's credibility on this was pretty lousy as well). So, even the most cooperative of countries on weapons of mass destruction face the regime changing machine that is the US in the 21st century.
So, why would Iran bargain with the US given the regime change mania? Can the US accept yes as an answer, as in yes, we will make a deal over our WMD? Perhaps the good news out of Syria's horrific civil war is that it might lead to some American restraint, which, in turn, might lead it to be a better bargaining partner. This might not solve the Iranian nuclear weapons problem, but it is probably a necessary condition for any progress, right?