I haven't heard yet about countries trading Bitcoin with each other but it might happen in few years from now.
It is not going to happen so far because now almost everyone country is accepting bitcoin and they are trading and investing in it, I am sure soon bitcoin will become worlds most wanted and the only coin to trade a trust worthy with it, because the development of trading is the only way to make the future economical progress of any country, so I think bitcoin trading is going to have a bright future.