Director of National Intelligence and Obama appointee Dennis Blair back in April replied to a Senator’s question that Iran is funding the Taliban:

“Iran is covertly supplying arms to Afghan insurgents while publicly posing as supportive of the Afghan government.” That includes the provision of small arms, mines, rocket-propelled grenades, rockets, mortars and plastic explosives. Ironically, Blair also noted that Iran has been assisting Afghanistan with developing their security capabilities through the “construction of border security facilities.” … (link)

It should be noted that Blair has been caught in a lie before, according to investigative journalist Allan Nairn. Nairn revealed earlier this year that documents revealed that Admiral Blair lied about a meeting he had back in 1999 with an Indonesian general in Hawaii. Blair claimed that he had heard nothing of a recent massacre in East Timor when he had met the general and offered him US support and military aid. Nairn revealed that unearthed documents showed that Blair’s aides had in fact spoked at length about the massacre which happened two days before the 1999 meeting in Hawaii.

I have always been skeptical of US claims that Iran is funding the Taliban. And it seems I am not alone, as Vikram Singh Obama’s advisor says that any such support is not “substantial.” In saying this, he sounds very much like fellow Obama advisor and leading authority on Afghanistan Barnett Rubin.
Now another authority on Afghanistan weighs in with a very interesting take on Taliban funding. Jean MacKenzie of the Institute for War and Peace Reporting is an extremely well-placed observer who has provided revealing commentary in the past:
Who is funding the Afghan Taliban? You don’t want to know
Jean MacKenzie

KABUL, Aug 13 — It is the open secret no one wants to talk about, the unwelcome truth that most prefer to hide. In Afghanistan, one of the richest sources of Taliban funding is the foreign assistance coming into the country.

Virtually every major project includes a healthy cut for the insurgents. Call it protection money, call it extortion, or, as the Taliban themselves prefer to term it, “spoils of war,” the fact remains that international donors, primarily the United States, are to a large extent financing their own enemy.

“Everyone knows this is going on,” said one U.S. Embassy official, speaking privately…

Up until quite recently, most experts thought that drug money accounted for the bulk of Taliban funding. But even here opinion was divided on actual amounts. Some reports gauged the total annual income at about $100 million, while others placed the figure as high as $300 million — still a small fraction of the $4 billion poppy industry…

The new feeling is that less than half of the Taliban’s war chest comes from poppy, with a variety of sources, including private contributions from Persian Gulf states, accounting for much of the rest…

Anecdotal evidence is mounting that the Taliban are taking a hefty portion of assistance money coming into Afghanistan from the outside.

This goes beyond mere protection money or extortion of “taxes” at the local level — very high-level negotiations take place between the Taliban and major contractors, according to sources close to the process.

A shadowy office in Kabul houses the Taliban contracts officer, who examines proposals and negotiates with organizational hierarchies for a percentage. He will not speak to, or even meet with, a journalist, but sources who have spoken with him and who have seen documents say that the process is quite professional… (link)