United States IRS has a webpage on Countries with Tax Treaties:
http://www.irs.gov/Businesses/Intern...aties---A-to-Z
In an article on Pokerfuse by Tax Attorney Brad Polizzano:
"US gambling winnings earned by residents of the following foreign countries are not taxable in the US: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Hungary, Iceland, Ireland, Italy, Japan, Latvia, Lithuania, Luxembourg, Netherlands, Russia, Slovak Republic, Slovenia, South Africa, Spain, Sweden, Tunisia, Turkey, Ukranie, and the United Kingdom."
I would still verify with the WSOP.