If bitcoin is hold as long term saving asset, it will raise such a question:
Current law says that when you have large amount of foreign currency gain due to exchange rate change, that is capital gain, should be taxed as capital income, similar to other type of capital gain
But that only applies when you cash out the bitcoin into domestic currency. If later you moved to abroad and invest/consume your bitcoins, I guess that country will not try to tax you, since for that country the foreign capital injection is always a benefit, if you try to tax them they will never arrive
Unlike other assets (stocks/bonds/real estates), which can only be traded on domestic exchanges and be taxed accordingly, if you want to tax bitcoin, you must tax it before the people leave the boarder, but that is not OK because at that time the coins have not been exchanged into any other currency thus does not generate any taxable event
So maybe it is not right to classify bitcoin as asset, an asset without boarder is in fact physically non-taxable
I have this question when I just went through an old report that swedish bailiff are trying to find people evading tax with hidden bitcoins
http://sverigesradio.se/sida/artikel.aspx?programid=1646&artikel=5983956