“This 152-year-old law forbids any American from issuing a check, note, or token that’s worth less than $1, and Bitcoin might just fall into that category, according to a report from the Congressional Research Service. The Stamp Act was written during the dark days of the Civil War, when inflation had pushed the value of the metal in coins above their official denomination. That put a serious pinch on the change supply in the United States, one that began to be filled by companies that issued their own paper versions of dimes and nickels, called ‘shinplasters.’ To stamp out competitors, it introduced 18 U.S.C. §336, the Stamp Payments Act of 1862. Violators face fines and up to six months in jail.”
http://www.wired.com/wiredenterprise/2014/01/stampact/