[d0ugie]

According to this Q&A from the IRS, yes, miners are on the hook: https://www.irs.gov/pub/irs-drop/n-14-21.pdf:

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. (...)

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.

[BrandonM]

That's only federal tax. I don't think Washington has state income tax. And generally only large cities have local income tax.