The idea is to weaken Iran financially, because 85% of Iran's export income comes from oil and 40% of gasoline used in Iran is imported (even though it is the fourth-largest producer of crude oil) because of a lack of local refining capacity.
Financial-futures analyst Gary Dorsch reports that, contrary to analysis in the press that holds warm weather as the cause of falling oil prices, the real reason is that an excess of 700,000 barrels of oil is being produced by OPEC countries.  Only Saudi Arabia has the spare capacity to bring market prices down.
Add to that the growing hue and cry about the rising "Iran threat" that one hears in the Gulf Arab states. The Saudi government, elites and Muslim scholars are issuing increasingly dire predictions about the growth of Iranian power; they are manufacturing hysteria about an "Iran threat". Even Yusuf Qaradawi, the famous Egyptian preacher with a slot on Al-Jazeera, is criticizing Iran for allegedly spreading sectarian strife in Iraq. 
During the final stop in US Secretary of State Condoleezza Rice's recent trip to the Middle East, in Kuwait City, where Robert Gates, the defense secretary, joined her, Arab foreign ministers from Egypt, Jordan and the six members of the Gulf Cooperation Council stood together to produce a united front against Iran.
In related news, Mujahedeen Khalq has released a list of more than 32,000 Iraqis working as agents of the Iranian regime for the Al Quds Brigade.
Oh wait, the U.S. struck a deal with MK very early on after the invasion.