Unless our society changes dramatically, a single currency will simply not be possible. Single currencies are only possible in areas which have a similar level of economic growth, and where free movement of goods, labour and capital balance out growth differences. Take Europe as an example - the Euro does okay, but countries can still get into trouble when their growth levels differ from the majority, as in this case they would normally set a different interest rate to adjust.
Italy is a big example. Through having a generally poor economy, corruption and bad politics, their economy has seriously stalled. Because it's growing much slower than the rest of the Euro area, it's finding it very difficult to cope (it needs a low interest rate to stimulate the economy) - there has even been talks of it having to drop out of the single currency, although I doubt that will happen.
The U.S.A manages to do fine with the single Dollar for a few reasons. Not only does it have complete free trade and free movement of labour between states, but it also has a unified language and culture, and broadly similar laws. Thus, free movement of labour is more of a reality than in the E.U, where most migration is from poorer countries to richer, and there isn't yet very high levels of labour between the main E.U countries (although this is present), mainly because of big language and cultural differences. Thus, capital and labour flows all America to largely balance itself out.
Unfortunately, the world is very, very far from getting to a state where a single currency is possible. We would first have to see the complete development of all third world countries, and complete free trade and free migration everywhere in the world for a significant period of time before global interest rates would be possible.
I believe that this could well happen one day, however I would suggest that we're still 200-300 years from that stage. However, it could happen faster - new wireless and renewable technologies are allowing poor countries to leap frog the need to build infrastructure, going straight to mobile phones, wireless technologies and de-centralised power generation. If Africa manages to get over its corruption and tribal differences then this could happen much sooner.