Interesting aspect, from what I know IRS classified bitcoin as a 'property' and not as 'currency'. In general the IRS is taking a cautious approach due to infancy of cryptocurrencies.
So I wonder whetehr IRS could tax BTC and if yes, then 'how' huge bitcoin stash could be taxed? We have to remember that law is not retroactive.
I said IRS assuming Satoshi is a US citizen. Although most likely he is/they are not, the same will apply to any country. Where I am from, I shouldn't declare bitcoins I have mined. However, when I am going to sell them, I should declare the income for taxation. Therefore, if I was to declare that I hold 1.000.000 btc, they will watch and monitor any step I make.