The trick in Argentina's case is that they have fixed the exchange rate to USD at an unfair amount and have enacted capital controls restricting the movement of currency into and out of the country. This is basically for the purpose of preventing people from trading their money into USD.

Brief Economist coverage here: http://www.economist.com/blogs/americasview/2011/11/argentin...

There was a decade in which pesos were directly convertible to USD at 1:1 exchange. That system collapsed when the government became insolvent.