The U.S. dollar is sort of the de facto world currency. Barrels of oil, for instance, are bought and sold in dollars.

That's changing, though. http://en.wikipedia.org/wiki/Iranian_oil_bourse

Some people speculate that keeping the oil trade based purely on dollars has been a major driving force in US foreign policy for the last decade. Certainly, it seems a very reasonable explanation why the Neo-Cons are constantly saber-rattling at Iran.