My comment to Ed:
Any fixed rate system is going to turn out to be deflationary rather than "stablizing," putting national policy in a bind in downturns. Is there going to be a lender of last resort? Who sets the conditions? Is this government body in control of the world's reserve currency elected and accountable, or it is composed of the world's super-bankers? The New World Order people would have a field day with this, and I suspect that a lot of the Tea Partiers now in control of the GOP are NWO'ers.
Sovereign governments would have to give up monetary sovereignty to join such a currency union even at the international level. Is the world an optimal currency area?
See also Dani Rodrik's "inescapable trilemma of the world economy".
Lots of issues here. Look forward to your thoughts on this.