if you just mine bitcoins, then that's zero income. once you convert the bitcoins to currency, then that currency would be applied as any other income, not capital gains
Fincen stated that bitcoins were a virtual currency and gave guidelines on how to regulate them. Basically it stated virtual currencies were to be treated as a commodity and arent regulated, except where you are involved in transmitting them to/from fiat, then you have to be regulated as a MSB. The same goes for miners, if you sell coins you mined then you have to follow MSB regulations, though i dont see how they would enforce that.