Technically to be 100% right, you should record the price after each block found with the pool you are using. Of course that is not going to happen (although would be a great piece of software idea..) so instead I mine all coins to a single address that I dump at the end of each month and record my basis as that. So you owe ordinary income on that part, then any fluctuation of price when you sell the assets would be capital gains.
This is interesting. The question is how the IRS will know that I mined bitcoin, if no pool reports it to the IRS. Many pools are not even in US, I doubt they will take the trouble to report to IRS, moreover the pool has no idea which country I live.