The coin act of 1965 says something like this
Section 102 states: All coins and currencies of the United States (including Federal Reserve notes and circulating notes of Federal Reserve banks and national banking associations), regardless of when coined or issued, shall be legal tender for all debts, public and private, public charges, taxes, duties, and dues.
What "legal tender" means is that if I owe you money, you cannot refuse my currency that has been defined as legal tender (U.S. dollars, in this instance) as payment for that debt.
It cannot be construed to mean that only U.S. dollars can be used as money. If both parties agree, bananas could be used to make a purchase or to settle a debt.
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http://en.wikipedia.org/wiki/Legal_tender