The U.S. and other governments could attempt to regulate and/or prohibit Bitcoin usage. While that could not shut down the Bitcoin system, it would force it underground. That would end the hopes and dreams of a mainstream Bitcoin economy, and would certainly destroy everyone's fantasies of wealth by driving the price back down to single digits.
... Any country that doesn't participate in the ban will suddenly see a big rise in exports because they accept bitcoins and no-one else does. ...
That's doubtful. At this point, the Bitcoin economy is no more than a single drop in an entire ocean. As I pointed out in another thread, the Federal Reserve prints 40 times the total value of all Bitcoins every month.
