Out of their own mouths

This is an excerpt of a report given to the Senate Armed Services Committee by the Director of National Intelligence, James Clapper, on 16 Feb 2012 and is the clearest indication that the US knows Iran does NOT have a nuclear weapons program:

We assess Iran is keeping open the option to develop nuclear weapons, in part by developing various nuclear capabilities that better position it to produce such weapons, should it choose to do so. We do not know, however, if Iran will eventually decide to build nuclear weapons.

Iran nevertheless is expanding its uranium enrichment capabilities, which can be used for either civil or weapons purposes. As reported by the International Atomic Energy Agency, to date, Iran in late October 2011 had about 4,150 kg of 3.5 percent enriched UF6 and about 80 kg of 20-percent enriched UF6 produced at Natanz. Iran confirmed on 9 January that it has started enriching uranium for the first time at its second enrichment plant, near Qom.

Iran’s technical advancement, particularly in uranium enrichment, strengthens our assessment that Iran has the scientific, technical, and industrial capacity to eventually produce nuclear weapons, making the central issue its political will to do so. These advancements contribute to our judgment that Iran is technically capable of producing enough highly enriched uranium for a weapon, if it so chooses.

We judge Iran would likely choose missile delivery as its preferred method of delivering a nuclear weapon. Iran already has the largest inventory of ballistic missiles in the Middle East, and it is expanding the scale, reach, and sophistication of its ballistic missile forces, many of which are inherently capable of carrying a nuclear payload.

We judge Iran’s nuclear decision-making is guided by a cost-benefit approach, which offers the international community opportunities to influence Tehran. Iranian leaders undoubtedly consider Iran’s security, prestige, and influence, as well as the international political and security environment, when making decisions about its nuclear program.

Iran’s growing inventory of ballistic missiles and its acquisition and indigenous production of anti-ship cruise missiles (ASCM) provide capabilities to enhance its power projection. Tehran views its conventionally armed missiles as an integral part of its strategy to deter — and if necessary retaliate against — forces in the region, including US forces. Its ballistic missiles are inherently capable of delivering WMD, and, if so armed, would fit into this strategy.

As you can see, they "do not know if Iran will eventually decide to build nuclear weapons". This implies that Iran is not doing so now. In spoken testimony to the Committee, Clapper was asked the questions

Sen. LINDSEY GRAHAM (R-SC): You have doubt about the Iranian’s intention when it comes to making a nuclear weapon?

JAMES CLAPPER: I do.

GRAHAM: So you’re not sure they’re trying to make a bomb? [...]

CLAPPER: I think they’re keeping themselves in a position to make that decision but there are certain things they have not yet done and have not done for some time. [...]