Probably the same rules as Apple holding its profits offshore - when you change it back into your local currency, you pay tax on it.
I think when you convert bitcoin to fiat you will be charged a fee. for the amount fee it will depend on each exchange that you use.
but it is not a tax by the government when you convert it to a fiat
Yes I think so. Taxation on bitcoin is not easy because the user is readily know. Britain and europe have been trying to provide clear rules on companies that fall within the scope of virtual currency such as exchange of virtual currency by minimize the anonimity of people who using the services of these companies.