it's called capital gains taxes, and it depends on the price that you bought the bitcoins at. if you bought at say $10, and it jumps to $1000, then you'd have to pay taxes on $990. the % depends on the itemized deductions, and how long you've held on bitcoins (in the US).
According to my accountant, Beetcoin is correct. You should consult an account for your own situation but technically if you did not hold it for a year it will be short term capitol gains, over a year is long term capitol gains. It does not matter if its Kittens or stocks, if you bought at one price and sold for a gain or loss it is a capitol gain or loss.
I am aware of the argument that BTC is not something you can hold in your hand but I will play it safe and just pay the taxes on it. I moved enough USD tied to BTC this year I need to do something regarding taxes.
I know BTC is fairly new to being reported as income or capitol gain but its better to pay it wrong and show the attempt than not pay it at all. I have a feeling after this year in the US we will have some kind of IRS guideline as how to handle the reporting and paying of taxes on BTC.
Much depends on where you are standing on the planet and where your assets are located.
