They are helping both the Sunnis and the Shiites:

Iran is supporting both Sunni and Shiite terrorists in the Iraqi civil war, according to secret Iranian documents captured by Americans in Iraq.
The news that American forces had captured Iranians in Iraq was widely reported last month, but less well known is that the Iranians were carrying documents that offered Americans insight into Iranian activities in Iraq.
An American intelligence official said the new material, which has been authenticated within the intelligence community, confirms “that Iran is working closely with both the Shiite militias and Sunni Jihadist groups.” The source was careful to stress that the Iranian plans do not extend to cooperation with Baathist groups fighting the government in Baghdad, and said the documents rather show how the Quds Force – the arm of Iran’s revolutionary guard that supports Shiite Hezbollah, Sunni Hamas, and Shiite death squads – is working with individuals affiliated with Al Qaeda in Iraq and Ansar al-Sunna.
Another American official who has seen the summaries of the reporting affiliated with the arrests said it comprised a “smoking gun.” “We found plans for attacks, phone numbers affiliated with Sunni bad guys, a lot of things that filled in the blanks on what these guys are up to,” the official said.
[…]
Rough translations of the Iranian assessment and strategy, as well as a summary of the intelligence haul, have been widely distributed throughout the policy community and are likely to influence the Iraq speech President Bush is expected to deliver in the coming days regarding the way forward for the war, according to two Bush administration officials.
The news that Iran’s elite Quds Force would be in contact, and clandestinely cooperating, with Sunni Jihadists who attacked the Golden Mosque in Samarra (one of the holiest shrines in Shiism) on February 22, could shake the alliance Iraq’s ruling Shiites have forged in recent years with Tehran. Many Iraq analysts believe the bombing vaulted Iraq into the current stage of its civil war.

It’s amazing that the Iraqis think that the Iranians actually want to help them. I hope that they learn about Iran playing each side off the other(maybe that’s why this article was published?).
(via)