The Iranians planned to employ Mexican drug traffickers to kill Jubeir with a bomb as he ate at a restaurant, U.S. officials said.
The logistics alone to this seem like a nightmare. From the use of Mexican drug traffickers (who might not even be able to get into the country, much less to DC, and run the added risk of being recognized and thrown in jail for the ignominious charge of marijuana possession), the fact that a restaurant is a rather iffy place to find the ambassador, liable to change at a moment's notice.
Not that it was ever much of a threat, anyway, since the United States Attorney has stated that law enforcement officers were guiding their sources the whole time, and no explosives were ever placed anywhere. In essence, we've arrested a man on the basis of what he might have been able to do if the CIA and FBI had let him.
As a result of all this, Eric Holder and Hillary Clinton have touted the need for further sanctions against Iran, because obviously not talking to them helps them come to our side, just the way social ostracism works in high school.
Throughout all my travels through the internet punditry and news sites, I've not seen any concrete proof of Iranian involvement, except the fact that Mansour Arbabsiar (a disheveled, disorganized, divorced man in never-ending trouble with a long list of creditors) might have had contacts with the Iranian Quds.
Overall, it just seems like a rather ridiculous assertion, and I hope nothing more than a few hurt feelings will come from it. For more, Glenn Greenwald wrote an excellent column on this subject. Enjoy.