It's actually somewhat of a difficult question, because digital currencies in the past were purchased outright with regular currency, not mined in the way that BTC are.

Any gain from sale of BTC that was purchased, not mined, is going to be a capital gain.  

Any gain from sale of BTC that were mined, is going to be essentially uncharted territory.  I think you can make the argument that since it is 'earned', in the sense that people purchase equipment, configure it to run certain software, pay for electricity, etc.  In this case, it would simply be treated as business income.  You could then deduct your expenses and arrive at net taxable income.  Some expenses would have to be depreciated over time, such as computers which would last longer than one tax year.

It is possible, however, that the IRS might take the position that since bitcoins are 'awarded' in a somewhat random fashion, that they might be considered 'gambling winnings'.  If this is the case, you wouldn't be able to deduct any of your expenses used to generate them.

Really it would be great if the IRS would issue a statement on this to give clear guidance, but bitcoins specifically are probably not even on their radar.