What Would it Take?

Do we have any economists in the house? Does anyone know what it would take to really go to a global currency? I mean it's fun to watch the half-baked conspiracy theorist go off, but I'm kind of interested in the real theory. A united world currency has to have benefits, right? No more exchanges at the airport, good for trade, lots of small, shaky countries don't have to worry about their currency going to shit. A world currency would have all kinds of benefits. One of best being 90% of the Fox News on-air staff would have their heads explode.