^Of course the US government could just outlaw bitcoins and cryptocurrencies for US citizens in much the same way it once outlawed individual possession of gold coins and bullion. That wouldn't be enough to stop some people - although it would certainly up the consequences rather drastically for those people unfortunate enough to get caught.
If things like bitcoins are going to succeed, they're going to have to make some attempt to reach an accommodation with current government backed currencies. If they're presented primarily as a way of evading regulation, or creating an underground economy, they're going to get stepped on pretty severely.
The one big problem with would-be Libertarian economic revolutionaries (from what I've seen) is their tendency to be overly romantic and somewhat naive. Direct confrontation is the one thing most global powers are well equipped to deal with. So while direct confrontation and nose thumbing may be cathartic, they don't do much to further social change - unless the target institution is already weakened and has lost the bulk of its own public's support. None of the major world governments are in that position yet - and likely won't be any time soon.
If cryptocurriencies are the wave of the future, those pushing for them are going to need to take a longer view, and play a much more mature political, economic, and social 'game' than they have so far.
I wish them well. But I don't hold out much hope because of the way so many of their advocates have been conducting themselves lately...