Although the US has condemned the assassination, we aren't being very strident in our denunciation of the perpetrator. What kind of "retaliation" can Iran effect? Plenty.

CNN:

Iran's President Mahmoud Ahmadinejad sent his condolences Friday to the families of Roshan and his driver, who also died following the attack, the state-run IRNA news agency reported. He pointed the finger at "agents of imperialism and international Zionism," a reference to Israel.

Other Iranian officials also blamed the killings on Israel and the United States, both of which have accused Tehran of pursuing a nuclear bomb -- a claim it denies.

Meanwhile, a newspaper aligned with Khamenei called Thursday for the Islamic republic to respond in kind to the killings of its nuclear scientists, suggesting Israeli officials could be targeted.

The editor of Tehran daily Kayhan, considered the organ of Khamenei's supporters, questioned why Iran should not "exercise its legal right to retaliate."

Mohammad Khazaee, Iran's ambassador to the United Nations, said the assassinations of scientists were intended to deprive Iranians of the right to peaceful nuclear energy.

"We believe that these terrorist attacks are supported by some elements -- especially within the Israeli regime as well as some quarters around the world," he said.

Iran says its nuclear program is aimed at producing civilian energy, not weapons. But it has rebuffed international demands to halt its enrichment of uranium, and the International Atomic Energy Agency says it has credible evidence that Iran has conducted weapons-related research.

The IAEA, which serves as the U.N. nuclear watchdog, reported in November that it can no longer verify that the Iranian nuclear program remains peaceful.

There are Hezb'allah cells in Mexico, and perhaps even in the US. If Iran so chose, they could almost certainly target some of our nuke scientists although why they would do so is a mystery. We are trying to kill their scientists because they have limited expertise. There just aren't that many nuclear scientists in Iran and it is difficult to replace those who are targeted. In short, we are able to degrade their program without bombing.

Needless to say, we have a lot more scientists and killing one or two would be a tragedy, but would not materially affect our programs.