I think one would claim it as misc income on realized gains ...those gains would equal that which was converted to fiat not that which is still held in a wallet (which might still appreciate) , and then only claimed for tax purposes if the income was above what was spent (on mining equip etc). One could also claim a loss if what you spent on mining equip was more than the realized gains.
Or claimed as capital gains...it's really up to interpretation of the law at this point, from what I understand.
http://www.bitcointax.info/ has some info on the subject.