It's already been stated multiple times here, but there are far too many "acceptable" examples of currency substitutes being employed by various parties in the US.  The US Government finds fault only with the Bitcoin because it is often used in underground trades for black market merchandise.

I will forecast that, although they will try to shut down .onion sites such as The Silk Road (which might've already been accomplished, given the look of it), they won't attempt to touch the Bitcoin.