If I win, I am required by Swedish law to declare my winnings, as according to them, all online currency holds RL value due to the ability to sell it online for RL money. Thus, even though someone might not have the intention of selling game money (and characters, recent addition), you still need to declare and tax them. This is true for all kinds of currency and money, be it online pocker or black jack, as well as WoW gold, FFXI Gil, etc. Even characters as they can be sold.
And no, it is not WHEN you sell them (if you do), its even if you don't have an intention to sell them. So, how much does 100 mil gil sell for currently? I'll need to know how much theoretical money the government might want me to tax.
I actually didn't know I need to tax my game money until I read it in the newspaper a few months ago. I dropped my jaw and was about to bust out laughing. Wonder how much I owe in tax evasion >.O
edit: It has actually upset a lot of people. In reality I need to declare everything I sell on the AH, or that drops from beastmen I kill. If you play online poker, you are required to tax every hand you play, not the money you win. Thus, you can actually be required to bay $10,000 in tax, even though you didn't actually win anything in Poker.