Here is a link to a PDF of a book excerpt that addresses many of issues involved in alternative currencies in the US:

http://www.smallisbeautiful.org/local_currencies/Book.pdf

Unfortunately, it's from photocopies and so not searchable.

It mentions, for instance, that issuing coins or notes that are fractions of US dollars is illegal and subject to criminal penalty.  This seems to signify that any foreign currency used in commerce in the US or any scheme such as Bitcoin might do poorly in Federal Court because they were/are issued with values that are fractions of dollars.  Of course, nobody much cares until these currencies aim to circumvent law or replace US currency.

Here's a nice resource:

http://money.stackexchange.com/questions/3582/are-there-any-countries-where-citizens-are-free-to-use-any-currency

and another interesting post:

http://www.quora.com/Is-Bitcoin-legal

and from aljazeera

http://stream.aljazeera.com/story/bitcoin-and-alternative-currencies

Cheers!