Well...according to a close relative who is actually an IRS person...it's income when you cash it for USD...NOT when you mine it. Same as gold bullion.
I would recommend not taking tax advice from that person. The IRS has declared that when you receive mining revenue it is treated as ordinary income, at the time you receive it, even if you don't convert it to USD right then.
There are financial reasons, if you are hodling, to pay your taxes now on coin mined now, that will save you a lot of money long term.
And since its different in every country, and way offtopic, we can quit the tax/irs discussion i guess?