I'm not concerned and neither should you be.  The points I make on this board are pretty much general "what ifs" when things get bigger.  I suppose pooled mining won't last that long.  However, if our IRS saw that the equivalent of 20+ million dollars comes into creation, you can be sure they want a piece.  In Las Vegas, if you win a large amount, they tax you right there.  I don't know if they'd let you go home with the chips on the promise to pay the tax when you come back and sell them to the cash cage.  They equate the win with money, just like the win of a car.  I don't know whether they'd tax the Bitcoins as cash, but they would impute a certain value to the Bitcoins and tax someone on that.  Now, here in the US, the IRS would realize that it was not completely feasable to tax the person who got the Bitcoins due to anonymity reasons, so I think they would tax mining pools here, theorizing that the mining pool actually took possession of the asset before passing it along, and have the mining pool do "withholding" just like an employer does with wages, or Vegas does with chip winnings.  Hopefully this flies under their radar, or we base the mining pool in Switzerland or the Grand Cayman Island.