Seriously, at this point, I'd believe anything. I was reading over the 9/11 thread here and downloaded one of the videos and I did see the conspiracy theory one, and for as crazy as it sounded, some of the parts actually made sense
Not to get too political, but I still think nuclear weapons is the main reason. I think the US sees the government in Iran as a terrorist organization (or at least one that the US suspects has aided terrorists).
Iran definitely seems like a problem. Just from the comments they've made--"the US will will suffer hurt and pain" and "there was no holocaust" is evidence enough that these leaders don't seem to be in the right state of mind to have nuclear weapons. That's why the IAEA was formed--to weed out countries with weak or suspect ethics.
However, I do also agree with the dollar depreciation scenario. The US dollar has historically proven to be unreliable in times of economic and global unrest. And if oil exporters needn't rely on the US dollar anymore because other currency has become more valuable/easier to use, then yeah, I guess the US has a reason to worry a bit.