You will still be able to trade Bitcoins for dollars in the US. Barter is 100% legal.
But if governments prohibit the possession and selling of bitcoins (much like illegal drugs), would you still be able to trade them?
(would they? I think so, seeing what bitcoins could be used for)
I'm not quite sure how barter would work for bitcoins, but whatever.
Prohibition on the possession of Bitcoins in the US is extremely, extremely unlikely. There is no reason they could outlaw Bitcoin that would not also apply to cash.
Every Bitcoin transaction is barter from the government's perspective.