List mining as either "Accounting Services" or "Database Services".  This is on the premise that what mining does is maintain the Bitcoin account books (the block chain).  Mining rewards are merely a data entry in the account book.  It becomes income for tax purposes when you get local currency or some other barter value out of your data entry.  The IRS calls this "constructive receipt".  Compared to an independent accountant who does work for other people, maintaining paper accounts or a spreadsheet and making entries for his own time are not income yet.  It becomes income when he gets a payment from someone that he can go spend elsewhere.

Like any business, you can deduct expenses for hardware, software, networking, and utilities.