A pair of Buffalo-area men face charges of attempted petit larceny and misdemeanor criminal impersonation for trying to use the private hard-money backed currency the Liberty Dollar to buy beer at a Buffalo Sabres game. (The original felony charges were dropped.) The U.S. Treasury Department, while stressing they are not legal tender U.S. of A cash money, does not think there is anything illegal about using them if your partner in the transaction wants to. See here for more accounts, with happier endings, of using, or trying to use, Liberty Dollars in everyday transactions.
Douglas Rushkoff writes of the "dangerous idea" of what he calls "open source currencies" (though he's mostly talking about localized labor-backed scrip, not Liberty Dollar's hard-money experiment) here.