Personally, I am somewhat skeptical of this as it is too sloppy for Iran. This isn't Iran's style. From the article:
Mr Holder described the plot as an "international murder for hire scheme" involving a plan to pay an assassin from a Mexican drug cartel $US1.5 million to carry out the killing.
If Iran wanted to assassinate someone, why would they use a Mexican drug cartel?
Why was the entire thing planned on US soil? That's quite risky. I mean, if you want to assassinate a diplomat, it'd be pretty easy to do, but everyone knows that US security, police, and intelligence are on the look out for even a mention of terrorism, thus your chances decrease greatly.
Finally, if Iran had succeeded, what would they have gotten out of it? How would it have furthered their interests? The plot would have been investigated and would have been linked back to Iran, thus they could have had Saudi Arabia and Israel wanting to attack them and everyone knows that if Israel attacks Iran, America is right there with them. Thus you'd have the world's superpower (US), a regional superpower (Israel), and a major regional power (Saudi Arabia) all bombing the crap out of your country. While the Iranian government may act crazy, they aren't stupid. Thus, I have to wonder why Iran would even do this. (Please note that I am not saying its impossible, but it looks a bit strange is all.)