I believe some country, in particular China will ban bitcoins just because it is something they cannot truly control. That will have some impact and to what extent I am not sure but probably reduce the success of bitcoins dramatically because 1 Billion people not being able to use bitcoins is a big deal.
Ithink that China will deny their own people use of Bitcoin because that allows freedom. They will have to deal with the rest of the world using it though. If they don't then they'll lose an awful lot of trade.