You may want to contact a tax specialist or CPA, but my understanding is that since online gambling is illegal your only option for taxes is to report it under "other income earned" on a tax form. It's like a catch 22 since it's illegal earnings, but yet the government also wants to tax those illegal earnings. I've heard advice of getting an account with an offshore bank and taking out cash from an ATM so that it's not as tracable if you were audited. You might lose a % with fees and such, but your best bet may be to keep it in cash so there aren't as many paper trails to follow. If you do end up paying taxes on it you would pay in the high bracket like a lottery winner, upwards of 35% I believe.
My advice, if you're really making that kind of money you're either good or lucky. In either case look into a trip to Vegas and keep everything legal so you can enjoy your winnings to the fullest without worrying about the FBI knocking down your door.