Well actually, some games like Second Life actually encourage players to use real life money. In those games, I'm not so sure whether or not a case could be made that real-world income shouldn't be taxed the way all other income is (right now, I'm willing to bet a lot of folks making money of secondary market transactions are not reporting them on their taxes

).
For a game like Vanguard, where the EULA clearly outlines the fact that the game and everything within it is the property of Sigil...and that discourages the secondary market and the infultration of real world currency...I would think could make a case against any involvement by the outside world.
It's going to have to be a many-faceted law if it does ever come about... hopefully games like Vanguard will still exist where those of us who want to can get away from the complications and crappiness that comes from real-world money being involved.