There's a special IRS form that casinos are supposed to accept which handles staking arrangements (though the WSOP does not recognize this IRS regulation). More info can be found in the following. Its an older link, but I think it still applies...
http://pokerfuse.com/features/in-dep...king-activity/
If you've already collected 100% of the winnings, you may want to consult an accountant.
There is also a thread somewhere on 2+2 that deals with lots of tax-specific issues. I don't know where it is, but you might be able to search around. If I come across it, I will post here.