My case may be of interest to you. I am an American living in Europe. The USA I believe is the only industrialized country in the world that taxes its citizens on money earned abroad. So theoretically, if I report my earnings as required, I would have to pay double tax on my poker winnings, if my total income exceeds my exemption (currently $100,000). Now that is really not that much - less than 70,000€. And remember that is total income (income from work + poker income). So, let's assume that I have a decent middle class job, or I am a capable poker player, or perhaps both. In this case, I will end up paying around 40% on my winnings here in Europe, and then around 35% on whatever is left in the USA!
I recently read an article on how it is becoming much more common for Americans to renounce their citizenship over this silly double taxation that nobody else in the world uses. I imagine that this is mostly wealthy businessmen working abroad, but it could apply to a wealthy poker player too.