vicklemos: There's a country called United States of America and its currency is sometimes displayed as USD (united states dollar).
Not AD (american dollar). I'm not making fun of you, but I actually saw that happen a couple of times, no joke.
So anywhere where "A" stands for America will have to "include" Panama, Bolivia, Uruguay, Guyana, Mexico, El Salvador, Brazil, Chile, Suriname, and many, many more... :P
A could stand for Africa, though :P
Well, in France we tend to call it "dollar américain" (american dollar). Which is a gross generalization about America, of course, and probably somewhat insulting for the other countries, but we tend to call the US and the continent by the same name. Too much US movies, cultural and political influence, news coverage, etc... so it tends to steal the show. I imagine that happens in quite a few other countries.
But on an international site, yeah, I always saw it spelled "US$", and A$ for Australia