In most countries tax is not put on Bitcoin itself but on exchange transactions, trading. The pressure for regulation is bigger and bigger because of anti money laundering regulations, tax evasions, illegal markets etc. But regulation doesn't necessary have to be the bad thing for Bitcoin and its users so we don't have to be afraid of it.
The possible things that government will do is to impose taxes on the way of exchanges and other merchants who accepts bitcoin on their businesses since its the only way to get money from those bitcoin users but getting directly into bitcoin its not really possible since bitcoin is decentralized and they cant able to access the system.