A question if you will. How does it work in the US? One must declare the bitcoins that he holds, or just declare the income when converting them to dollars?
Here in 'merica you would only have to claim your income you made from selling Bitcoins, the Bitcoins you're holding are completely worthless in the eyes of the IRS, they can't tax it. Which is why it is a good thing to pay for things using bitcoins.
I don't think that would be true in all cases. I might go with that if you bought 100 BTC at $100 and sold 100BTC at $200 however if you mined those same coins you would at best fall under the Uniform Capitalization Rules and since my cost would be included in determining Cost Basis the "income" if any would be what I determined much of it to be. At the same time if I traded those BTC for say silver I'm pretty sure there is no "gain" to be taxed. A Capital Gain is when you buy an asset at one price and sell it at a higher one. Income is derived from employment. BTC falls under neither unless we start stretching the terminology and laws.