Eli Lake has some details on the alleged plot:

Officials told The Daily Beast that President Obama first learned of the alleged plot in June, as undercover agents from the Drug Enforcement Agency were working on a sting against an Iranian American accused in court documents Tuesday of trying to pay Mexican gangsters to murder Ambassador Adel al-Jubeir…The plot, according to an indictment unsealed Tuesday, was approved at the top levels of Iran's elite Quds Force, an organization the United States has accused in the past of aiding the Taliban in Afghanistan and plotting lethal attacks on U.S. soldiers in Iraq. The Quds is an arm of Iran’s elite Revolutionary Guard tasked with carrying out the Islamic regime’s agenda outside the country.

Ken Pollack worries that this attack marks a sea change toward aggressiveness in Iranian foreign policy. Daniel Serwer thinks the right response is ratcheting up international pressure on the nuclear program and calling Ahmadi's "bluff" on negotiations:

How do American diplomats make nice with Ahmedinejad while announcing to the world that Iran’s security forces have been plotting murder, even mass atrocity if one version of the alleged plot had taken place, inside the United States?  But it is precisely at a moment like this–when Iran is going to find itself weakened and isolated–that the international pressure might be sufficient to force progress on the nuclear issue, with the added potential benefit of further fragmenting a regime whose president and “supreme leader” are already on the outs.  Maybe taking up the offer privately, cautiously and conditionally would work too.

Robert Haddick focuses on the implications for our ability to deter Iranian attacks.