Alternative currencies are already legal in the US. If I print up a zillion Foobucks and you agree to take them in exchange for goods and services there's nothing illegal about that. But if it's a taxable transaction then taxes will be due in real money.

Alternative currencies are legal but with serious limitations that make it almost impossible to compete with the official currency (see http://www.quora.com/Is-Bitcoin-legal).

So long as they don't look anything like dollars and you don't ever claim them to be legal tender. (Which has a different meaning in the UK.)

http://en.wikipedia.org/wiki/Liberty_Dollar