I am an accountant and at least for my US mining clients we believe bitcoins are taxed when the coinbase is complete.  This has been helpful to some as they have some significant expenses that they would have lost had they not have had the income to offset those expenses -  from the computers purchased to an allocation of home electrical use and even, for some, depreciation of their home office where their mining activity is conducted.

mgio is correct, a 'business license' or incorporating is not required.  Either or both may help evidencing that you are mining as part of a 'trade or business' as opposed to an 'activity for profit.' The difference between these two is a bit much for a board discussion but your accountant will know what it is and how, in your particular circumstances, it impacts your deductions and your taxes.

No specific IRS ruling says your coinbase award is taxed when you receive it.  We expect the IRS to say so.  Some know some tax people point to gold miners who do not report found gold as income.  This had more to do with the fact that you could not value the gold found because you did not know either its purity or how to reduce it to being a marketable quantity.  Had you been mining and happened on a gold coin, you would be taxed the moment it was found on it's value as the value was reasonably determinable.

So the value of the coinbase is reasonably determinable.  We have the indexes for bitcoin, and while you might argue each index is different, some higher, some lower, and whether you might use daily average price, or bid/ask as of the time the block was added to the block chain or the coinbase was spendable at 99 confirms, I would only suggest that once you chose a way to value it - stick with that method. 

Hope this helps.  Bottom line is..... tell Bill to go see his accountant.  20 bitcoin may be a marginal amount of money if earned in the earlier part of last year.  Tell bill to let his accountant determine if what Bill was doing was a 'hobby' or 'an activity for profit' or a 'trade or business.'  It's a determination that is technical but quick to make.  Your expenses may offset much of the income if the bitcoins were mined before March last.....

Oh and BTW - 20 bitcoins.... nice!

bob