Actually in another thread (don't remember which) I actually suggested a radical idea. Strike a bargain with Iran that makes it come under US protection as far as nukes go. Remember a hard line KSA is also under US protection. Egypt, Jordan all of whom fought against Israel once are now jolly good with each other under one common friend.
The bottomline would be: What does US really want? And what does Iran really want?
Does US simply want a non-nuclear Iran? or Does US want to replace the regime in Iran? or Does US want to simply balance the powers in middle-east? or Does US wants to prop up the Israel? or Does US have other objectives?
One can never be sure given the fate suffered by Iraq and Libya...
As for Iran: What does it really want? Does it simply want to have a deterrent? Does it want promise of no harm to its security? or does it have other objectives?
I think Iran wants nukes not only for deterrent purposes but for other political reasons in Middle-East. Internal politics of Iran also has to be kept in mind(to know how far radical ideas like yours can work).
I think Iran wants to be the hero of Middle-East and Muslim world by acquiring Nukes. Perhaps, once they acquire nukes, they maybe emboldened to go after the Israel.
So, the point is that without knowing exactly what is driving US and Iran, no mediation can be done.