In the Notice from March 25, 2015, the IRS issued some pretty specific answers to FAQ on mining virtual currency:
Q-8: Does a taxpayer who “mines” virtual currency (for example, uses computer resources to validate Bitcoin transactions and maintain the public Bitcoin transaction ledger) realize gross income upon receipt of the virtual currency resulting from those activities?
A-8: Yes, when a taxpayer successfully “mines” virtual currency, the fair market value of the virtual currency as of the date of receipt is includible in gross income. See Publication 525, Taxable and Nontaxable Income, for more information on taxable income.
Q-9: Is an individual who “mines” virtual currency as a trade or business subject to self-employment tax on the income derived from those activities?
A-9: If a taxpayer’s “mining” of virtual currency constitutes a trade or business, and the “mining” activity is not undertaken by the taxpayer as an employee, the net earnings from self-employment (generally, gross income derived from carrying on a trade or business less allowable deductions) resulting from those activities constitute self- employment income and are subject to the self-employment tax. See Chapter 10 of Publication 334, Tax Guide for Small Business, for more information on self- employment tax and Publication 535, Business Expenses, for more information on determining whether expenses are from a business activity carried on to make a profit.
You can read additional answers here:IRS Notice 2014-21