This whole incident has been quite sketchy. Not only the lack of information (if it was an attack or crash), but several other points. Here's my (very crazy and hypothetical) take.

I don't think terrorist organizations acted alone on this. First, look at the victim: a Japanese vessel. I understand that the Japanese are often considered to be a "Western" power, but how many times have we seen Islamic terrorism call for violence against Japan? Not much (there have been a few times). I would imagine that an AQ offshoot would rather target a European or American vessel instead of a Japanese. Second, I think that if terrorists are attacking the Strait, then they are being supported by Iran's Qud's Force (The Revolutionary Guard's foreign unit) with weapons, training, and intelligence. Third, I think that the attack's purpose was to send a message. Iran seems to be flexing its muscles by demonstrating how rapidly oil tankers can come under attack. This "attack" was light enough to attract attention, but not heavy enough to cause a massive reaction by the West with force. Simply, with the recent "rush" by Iran to begin the fuel swap, the attempt on Ahmadinejad's life, and the procurement of advanced AA missiles, tensions in the area are tightening. Iran wants to remind everyone how fragile those can be.

So, that's my take from my crystal ball. I doubt that it is completely correct...hopefully the "attack" was an accident.