No source, no reality.  Article 1 sec 8 states what the US Gov *CAN* do, and specifically it says that it CAN MINT COIN, that it can regulate the value of that coin.  It does not give the US Gov *ANY* legal standing to make ANY other form of currency, trade, barter, etc, illegal.  Period.  Read the constitution.  That which is not EXPRESSLY allowed is forbidden -> http://en.wikipedia.org/wiki/Everything_which_is_not_forbidden_is_allowed it applies specifically to the gov, if they're not EXPLICITY, DIRECTLY allowed something, then it's forbidden.  Basically, the most they can try to do is say "bitcoin is worthless here in the US in terms of the dollar"  -- they can say that until they're blue in the face, but if private entities accept/trade it, well, nothing they can actually do.  Just like now.