Reuters (and only Reuters) has the story:

The United States believes that Iran has supplied arms to insurgents in neighboring Afghanistan but top advisers to President Barack Obama said on Wednesday that the information was conflicting and any threat appeared unsubstantial.
Shi’ite Iran is not a comfortable ally of the hardline Sunni Taliban, but analysts say Tehran may be providing some support to tie down and irritate U.S. forces in Afghanistan.
Earlier this year, the U.S. commander of international forces in Afghanistan accused Iran of supporting the Taliban but said he had not seen the introduction of sophisticated Iranian military equipment of the kind that was sent to Iraq.
“We get conflicting reports on that,” Richard Holbrooke, the U.S. special envoy to Afghanistan and Pakistan, told a panel organized by the Center for American Progress, a Washington think tank, when asked if Iran was supporting the insurgency.
Holbrooke’s senior defense advisor, Vikram Singh, said: “Certainly, the Iranians have in the past provided some arms to some groups inside Afghanistan. I do not think it has been viewed from a defense perspective as a substantial effort or a substantial threat.”
A U.S. counterterrorism official, speaking on condition of anonymity, said Iranian assistance came mainly in the form of arms, rather than direct training of militants
“There’s reason to believe that Iran is supplying arms and other materials to insurgents in Afghanistan, including the Taliban,” the official said.
He provided no details about the types of arms.
Holbrooke said Tehran had a “legitimate role to play in the resolution of the Afghan issue.”
“They are a factor. And to pretend that they’re not, as was often done in the past, doesn’t make much sense,” Holbrooke said, but added: “We don’t have any direct contacts with them on this.”
Drug addiction is a major problem in Iran and Holbrooke said “those drugs are coming across the Afghan border and it is a major concern to them.”

Obviously, this is a divergence from the Bush administration, which often played up the role of Iran in Afghanistan.