Laura Rozen on her Yahoo! news blog.

[...]

In response to questions about the various theories flying about, several current and former American officials told Yahoo News Sunday on condition of anonymity that Israeli Defense Minister Ehud Barak had last month issued a request to the Pentagon that the exercise be postponed. The United States did not seek the delay--and American sources privately voiced concern that the Israeli request for a postponement of the exercise could be read as a potential warning sign that Israel is leaving its options open to conduct a preemptive strike on Iran's nuclear facilities in the spring. Thus, the concern went, it may not want 5,000 U.S. troops on the ground in Israel in April and May, as had been scheduled for the exercise.

Andrew Sullivan writes:

I do not know the life, background or motivations of one Mostafa Ahmadi Roshan, who was killed, along with another passenger, when a motorcycle rider out of a Bourne movie stuck a plastic explosive on his car door and blew him to smithereens. What I do know is that he was a scientist working, we're told, as a procurer in Iran's nuclear power/arms program. Does he make the decisions in this theocratic tyranny? Is he responsible for the policy? Maybe he is an adamant Khamenei supporter. Maybe not. But he has been assassinated by someone. How should we respond?

Here's how Rick Santorum responded to these kinds of killings:

On occasion scientists working on the nuclear program in Iran turn up dead. I think that's a wonderful thing, candidly.

Guardian report.

An Iranian university professor working at a key nuclear facility has been killed in a bomb explosion, the latest in a series of assassinations and attempted killings linked by the country's authorities to a secret war by Israel and the US to stop the development of what Tehran insists would be a peaceful nuclear capability.

Mostafa Ahmadi Roshan, 32, a chemistry expert and a director of the Natanz uranium enrichment plant in central Iran, died after two assailants on a motorcycle attached magnetic bombs to his car, according to the semi-official Fars news agency.

new York Times report.

The imminent opening of the enrichment site -- the Fordo plant, near the city of Qum -- confronts the United States and its allies with difficult choices about how far to go to limit Iran's nuclear abilities. The new facility is buried deep underground on a well-defended military site and is considered far more resistant to airstrikes than the existing enrichment site at Natanz, limiting what Israeli officials, in particular, consider an important deterrent to Iran's nuclear aims.