Some stamps were used in place of coins by the Union during the American Civil War (http://www.coincommunity.com/forum/topic.asp?TOPIC_ID=95096), although I can't tell you if stamps on their own were ever legal tender.

I think you might be confusing the Terry Pratchett book "Going Postal" where stamps are used as currency since they are easier to transport than coins and had "worth" behind them. Since they promised X amount of travel i.e. 10 cents worth of travel it could be seen that a stamp had value so people used it in the place of currency.

However TP uses many real life examples in his books so it might have happened.