Without further evidence, the alleged Iranian plot to assassinate Saudi Arabia's ambassador to the United States is rather hard to believe. See Glenn Greenwald's take, for example, to appreciate the need for skepticism about U.S. claims–and the eagerness of many elite pundits to take the government story more or less at face value.
Jim Lobe's piece on how Iran experts are reacting is worth reading too. Juan Cole's post has a provocative, almost unbelievable headline–"Is an Iranian Drug Cartel Behind the Assassination Plot Against the Saudi Ambassador?"–but then again, the Official Story is pretty out there, too.
One can never underestimate the ways elite media can be spun by official sources, as this anonymous quote in the Washington Post today (10/12/11) demonstrates:
"There's a question of how high up did it go," said an administration official who spoke on the condition of anonymity to discuss internal White House thinking. "The Iranian government has a responsibility to explain that."
Under normal circumstances, a government accusing another government of a criminal terrorism plot would have to demonstrate that it has the evidence–not the other way around. I guess I'd want to remain anonymous, too, if I was going to say something like that to a newspaper.