On the subject of the "Iranian conspiracy", well, it all smells to high heaven. The Saudis and the Iranians are maneuvering against each other across the Middle East right now and, with the recent unrest in Shiite areas of the Kingdom, Iran is probably trying very hard to turn the geopolitical situation to its advantage. The biggest problem for me is that I don't really believe that the Iranians would choose to do such a bold attack. Killing Americans in the capital. The chance they would be caught was high. If they were successful the chance of war would be very high as well. And while Mexican drug cartels are very bad people, they too have a history of avoiding the deaths of civilians in the United States. It's bad for business. However, the Zetas are the craziest of the bunch, and they're losing the war against their rivals, the Sinaloa cartels and a few others. I strongly suspect, with reason, that the US government has made a secret defacto alliance with the Sinaloa cartel to destroy the Zetas, because the Zetas pose a bigger threat to the security of Mexico. Perhaps Commandante Z-40 decided that he would even the odds and get a state backer of his own, he's losing the war fast anyway.
EDIT: Upon re-reading the latest news, it doesn't seem that the Iranian guy who was arrested ever actually made contact with a Mexican drug cartel, rather, he talked to a DEA informant who is a figure in the whole narco-world in Mexico.
Fast and Furious is only the beginning if you really want to look into just how dirty and dishonest the "war on drugs" is and has been for a long time. I hope someone pulls on that string and reveals it all.