I suppose I could understand how the United States is different but the original question was directed at all countries in the world. The United States is different in that the USD is the country's number one export. A move to bitcoin would end the monopoly of the dollar. Bitcoin would create an even playing field for ALL countries around the world. This could scare the US but if the rest of the world started using it, I don't see how the US could stop it and would eventually have to join the party.
It would not matter if the rest of the world used bitcoin because a currency is intended to be used internally first. Trying to force someone like that is just pointless.
Blows my mind that peeps think it can not be taxed or monitored by a government. It can be taxed by regulate any point of sale on a countries soil. It can be taxed and monitored at any exchange on their soil. It can be taxed and monitored when transferred to any bank on your soil. See dwolla/Mt. Gox. They can and will control it within their borders and it's a necessary step for widespread adoption as a payment and an investment. They only way it can't happen, is if you never spend it on a service or good and you never exchange change it for local currency. In which case, your just sitting on a number that makes you happy. It's going to be a quite a while before you can do end to end BTC, if ever.
Bitcoin by design cant be monitored and if you exchange in your wealth with services that also cant be monitored they are essentially gone. After that you can use your separate ID (your bitcoin wallet) as your identification for buying goods or perhaps currencies without ever using your actual identity.
This is exceptionally easy to do since Bitcoin is an internet based currency not limited or defined by country borders.
One example is if i went to a bar and then payed them with a fraction of a bitcoin. Nowhere does my identity get recorded and the transaction is keept anonymous. Btw there are bars like this already.