Wow, just think about it how much money will go to Obama... Now IRS may have the best interest to keep Sotoshi safe, whoever he/she is. 
How it has not been converted to FIAT so they have no way to get him for taxes.
But in theory, it is taxable income. Just like property tax, you will pay eventually. That is what I am saying, before Sotoshi pays, irs has the best interest to keep him safe.