BitCoin is just too similar to Stock Market.
You buy X shares, even fraction (decimal) of a share/coin.
Then you sell, and calculate your loss or profit.
One thing that makes it different is you can mine coins, which you can't mine Stock Market.
How would you calculate a loss when mining, save your utility / electric bills?
Most I have spoken to (I am not a tax expert) feel that mined coins would be treated as income minus the cost associated with acquiring them. One would probably need to record the present value in what ever currency your country uses to fix a value when you take ownership. You could probably get away with some type of average per month for valuation or even quarterly; there is no clear precedent. I think the the IRS in the US would be flexible on the accounting method.
Say you mined 20 coins in January - the average price was $100 and the amortized cost of your mining gear, electricity etc was $300
20 btc x $100 = $2000 - $300 = $1700 net income
At the end of the accounting period (if you calculated monthly) you would have realized a net income of $1700 - (on your tax return - again i am not a tax expert, you would probably claim $2000 income then itemize the expenses.
If you later sold the 20 btc for $3000 then you would most likely also be responsible for $1000 in capital gains taxes.
