Well, the problem lies in the fact that unless you get a W2G, you can't really claim it on your taxes. Even though there's a spot on tax forms to claim gambling winnings, it pretty much applies to casinos in the USA that issue a W2G if your winnings are over a certain amount.....
After my miracle run in Vegas during a trip in 2007, I was in the black at all of the Harrah's properties to the tune of > $6,000, and they never sent me a w2g, as you can see in the picture below. So I never claimed those winnings...
As far as offline poker cash games go, there's no real way for them to know if you're ahead or losing at the end of the year. But if you have large tournament winnings, then those are definitely taxed.