I'm quite sure tax laws vary from one country to another. What is commonly shared among all countries perhaps is that income tax is being imposed to all. Which means that any income earned through legal means will be taxed. I'm afraid that includes earning from Bitcoin trading. However, there are differences on the limitations. Countries vary from one another on how much is a taxable income.

As regards Bitcoin income in particular, many countries' tax bureaus or agencies might probably find it very puzzling on how it would be computed, monitored, and assessed considering that many of them have even yet to understand what Bitcoin is. I myself am not paying taxes on my income through Bitcoin because there is not even a law regarding it. Bitcoin is not even considered a source of income.