What you pay tax on is the amount you change into FIAT. It is simply NOT considered capital gain, or bloomin' anything else.. might as well start trying to declare World of Warcraft gold or Eve online currency (can't remember what that's called) as your 'capital gains' ..
I can only speak to the USA, but converting it into something of value other than USD should still be a taxable event I believe. It'd fall under barter most likely:
http://www.irs.gov/taxtopics/tc420.htmlThat is, I pay you (as a barter since bitcoin isn't an actual legal tender, yet?) X bitcoins for a car worth $15,000 USD. That places my mined bitcoins at a value of $15,000 USD with a cost basis of zero, for a $15,000 realized gain.