So if you made 100 BTC last year and it's sitting in your wallet.dat, how much do you send to the IRS? Do they even have a receiving address for BTC?
Here's one interpretation:
We need some more data, so let's assume the 100 BTC was earned by mining two bitcoin blocks. One on June 30, 2011 and the other on November 1, 2011.
Record this as ordinary income of
50 BTC * $17 (price of bitcoins on June 30, 2011) = $850
50 BTC * $3.10 (price of bitcoins on June 30, 2011) = $155
Total $1,005
So whatever your tax bracket is (let's assume 25%), then your tax line would include $251 of tax.
Now if you want to offset the $1,005 with hardware & electricity costs, I think that's reasonable (and why I haven't declared any income yet). I'm not sure if you'll need to setup some sort of LLC to do that or not.
Disclaimer: I'm not a tax accountant, and do not rely on these estimates or interpretations as they are a brainstorm of thought and have not been verified by a qualified source.
I think this would apply only if you sold BTC for fiat - and in that case, the exchange rate at the time of sale would apply. If you simply hold on to your mined coins, there is no tax involved.
So say you cultivated 10,000 bushels of corn on June 30, 2011, someone said this is considered a "taxable event" and you would need to pay taxes on the market value of that corn.
~do not use this as tax advice~
Say you came up with a patentable and marketable idea, and kept it to yourself. Is this a taxable event? Ich don't think so. Say you painted a painting, even framed it, but never took it to the gallery for sale; instead, you kept it at home. Taxable event? Not in my country of residence.
A related thought: If Bitcoin is treated as a commodity, then in most jurisdictions each and every BTC transaction will include sales tax or VAT.