Actually the current guidelines are pretty clear. Mining is income. When you receive your coins from mining, that's a taxable event. Current value - mining expenses = income. After that any gains or losses are unrealized until you sell, creating another taxable event. Current price - cost basis = capital gains. Since bitcoins were worthless when Nakamoto was mining, he has no tax bill there. No taxes are owed for simply holding the coins. If he sold today, he'd be taxed at the long term capital gains rate of 15% for 100% of what he sold.

EDIT: but of course I'm making the silly mistake of assuming he's an American.