The option of bringing in a trusted third country into the mix is an interesting one. Turkey and Iran certainly have been improving their relations in recent years and Turkish Prime Minister Recep Tayyip Erdogan recently returned from a very successful trip to Iran, where he met with President Mahmoud Ahmedinejad and even the country's supreme leader, Ayatollah Ali Khamenei. The question is, though, if Iran truly "trusts" Turkey, in the sense that both are, at the end of the day, regional rivals and Tehran might be hesitant to enter into an agreement that gives Ankara an added amount of leverage over it (for more background on this, take a look at this previous post). From Ankara's perspective, if the Turkish option is accepted, then it would certainly validate Turkey's recent push to reach out to Iran and improve relations, despite some of the criticism that has led to.
[UPDATE -- Reports from Iran say Tehran has rejected the proposal of sending its uranium out of the country.]