Bitcoin miners will have to report their earnings as taxable income with a value equal to the worth on the day it was mined. If they mine as part of a business, they would have to pay payroll taxes as well.
IRS assumes you have to pay taxes twice. First time it's income tax when you get rewarded with bitcoins from mining, and then capital gains tax when you sell it (if it gains at all).
What is controversial, if you mined 1 BTC when bitcoin price =$1000, you owes let's say $350 in tax. If you don't sell mined bitcoins immediately, and then it drops to $200 per bitcoin, you still owes $350 in income taxes and suffers losses
Of course you may deduct up to $3000 from capital loss, but it won't help if you mined 20 BTC ($7000 income tax minus $3000 allowed capital loss deduction)
Everyone would agree it's absolute idiocy.
Probably it's better to omit the fact of mining bitcoins and just report that you bought it for cash and then pay capital gain taxes. They don't have brain scanning machines yet, so it's pretty safe strategy
P.S. i'm not professional lawyer or accountant.