Lets say I mine 1 btc on a pool and the pool sends me the BTC. I have a record of the transaction being sent to my wallet. I can then look up the value on bitstamp at the time I received the BTC, lets say it's worth $580. The IRS says I'm taxed on the $580 as "income" minus any expenses such as cost of miner and electricity. Now I wait 6 months and the coin I mined is only worth $200 and I sell. The question becomes, have I just realized a $380 capital loss to adjust income? This is the big question I have from reading the IRS FAQ.
You can deduct 3000 per year of capital losses against regular income, the rest gets carried forward. You can also use the loss as an opportunity to realize tax free gains by selling other appreciated assets. There are some edge cases where this causes a problem (for example, you have a huge loss December 31 and a huge gain the following day) but for the most part it is fairly reasonable.
If you don't understand these things learn or talk to an advisor.