World political (in)stability is such that I don't think one world currency could stand up to the fluctuations. If every country in the world voted to use the same currency (like Zimbabwe uses the US dollar), that would be one thing, but it would have to be a decision of the people, and even then, I doubt you could get them to agree on one currency. Some would want the dollar, some the Euro, and some might want German Deutschmarks! You just never know.
Unless it's a completely voluntary move by the country's residents, I don't see a one world currency working too well.