Well, I finally met a person who believes that the WTC attack was a conspiracy by the Mossad to create a pretense for war against Islamic countries. I knew these people existed, but I hadn't encountered any of them in the flesh until this weekend.
Interestingly enough, the conspiracy theorist in question was not a Muslim; he is a young Eastern European Catholic who has lived in this country for a few years. His "reasoning" was rather interesting: he asserted that crashing the planes into the towers was an act of aeronautical skill that only "a few top pilots in the world" could accomplish. "Who has those kinds of pilots? The Mossad!"
I saw no point in getting into an argument with him (who I'm somewhat acquainted with, and who is otherwise a perfectly nice and seemingly rational fellow), so I just smiled and nodded. He comes from a rural area in one of those countries that may not have liked being overrun by the Germans, but which enthusastically participated in the liquidation of their Jews... so I guess I shouldn't be surprised that he soaked up a certain amount of Zionist-Bankers-Rule-The-Worldism that may still be sloshing around in the culture there.
But that doesn't make it any less depressing.