I'm pretty sure that alternative currencies are illegal in the united states -- wasn't their a big fiasco with the FBI raiding the people making liberty dollars?

I think this makes it illegal: http://www.law.cornell.edu/uscode/18/514.html

Also, there may be tax issues. If you have an alternative currency and the us dollar is inflated do you have to pay capital gains taxes on the growth in value of the alternative currency?

There are also probably issues with money laundering -- in other words I don't think we can say that they exist, without a caveat about the legal limbo in which they exist.