Currency for money is a bit like timezones for datetime, so it's a good idea, not to add it to the base type interface. However, the interface should be extendable to include currency information.
Currency is much worse than timezones -- once you are interested in exchange rates, you need to know when to calculate the exchange rate (as well as other parameters such as which exchange rate).
So please let's keep the currency out of the money type; it's utterly application dependent what to do with that information.
--Guido van Rossum (home page: http://www.python.org/~guido/)