The Las Vegas Review Journal has a short article on the fact that online poker players must pay taxes. The article notes that winning players must pay taxes; of course, both winners and losers are supposed to include their winning sessions on their tax returns. There is one minor error in the article: Freeroll winnings of $600 or more should be reported on a Form 1099-MISC rather than a Form W-2G.
Both Brad Polizzano and I are quoted in the article. One point that I made with David Ferrara (the writer of the article) is that proposed legislation legalizing and regulating online poker in other states (than Nevada) explicitly requires reporting wins and losses to state tax agencies.