Umm, the answer to your question may depend on your jurisdiction, in any case:
Sold 10 BTC on Jan 1, 2013 at $100/BTC, you realized a capital gain of 10 x $100 = $1,000.
This capital gain needs to be declared in your income for the taxation year 2013.
However, Dec 31, 2013 you buy 100 BTC at $10/BTC = $1,000, so you lost $1,000. Depending on your jurisdiction you may deduct this against your capital gains for the year.
Therefore for the taxation year 2013 you made $1,000 by selling 10 BTC, and then lost $1,000 by buying 10 BTC.
This is definitely NOT true in the US. I can't speak for other countries.
Buying an commodity like bitcoin is NOT a loss as the bitcoin has value. Namely the value is what you bought it at! You can only realize a loss when you sell it for an amount less than what you paid. Or you realize a gain when you sell it for more you paid for it. The price you buy the bitcoin at is used as the basis for calculating your future gains (or losses).