EDIT: I saw another thread where someone had the idea of claiming earnings as "gambling" only if over $5000... Since Bitcoins are not considered a true currency by the US, couldnt i claim that i was gambling by using GPU power to harvest coins and holding on to them? https://bitcointalk.org/index.php?topic=153202.msg1627264#msg1627264

The question you have to ask yourself is whether there is any way your holdings in BTC can be linked back to you.

• If you have been solo mining and never moving your coins, you will have in your wallet as many new private addresses containing 50 / 25 BTC each as you found blocks. These cannot be linked back to you as long as you don't send them to an address that you have used for a transaction under your real identity, or any identity that could be traced back to you (you have to assume that the IRS in 5 or 10 years may have a much better technology to analyse the blockchain + information from exchange and large merchants they could use to connect the dots).
• If you have been mining using a pool under a pseudonym while making sure that nothing was leading back to you (email address included), and you never transfered the payout to an address that you have used for a transaction under your real identity, or any identity that could be traced back to you, the only existing link is the IP if the pool logged it and still have it in X years. That's a very very weak link, and unlikely unenforceable.

So if the answer to the above is : "No, the coins I mined cannot be linked back to me at this very moment", and if you have not been declaring your mining activity in any way, then you should be safe in regard to IRS as long as you make sure not to create the missing link that would allow them in the future to prove that you had hidden holdings that led to undeclared realized capital gains.

Now, if you have such coins that can't be traced back to you, and you want to redeem them for cash, there are a few ways you can do that while remaining under the radar of IRS:

• OTC trading: "virgin" blocks of 25 BTC / 50 BTC sell for a premium on the off-exchange market. Make sure to do the transaction in person and in cash, and don't give your real identity to your counterparty.
• Sell stuff to yourself for Bitcoins on ebay, with "in person delivery" so you aren't supposed to know the real ID of your buyer. Make sure the proceed remains within the limit set by IRS for private selling as individual. And keep all the records.
• Send the coins to a (close, trustable, and equally tax dodgy) relative who doesn't have taxable income, or a low taxable income, and have this relative withdraw the coins under his / her real name, in amounts high enough to be effective, but low enough to remain under the tax threshold. Make sure this relative *declares* this income and effectively receive tax clearance. Now the amount is clear from taxation, and you can have the relative offer it to you either officially using a bank transfer (if under the yearly limit set for donation / gift), or withdraw it as cash and give it to you in person. You may want to prepare a scenario to explain why your relative got that free money coming from nowhere in first place (sold something in-person for Bitcoins for instance).
• Send the coins to someone who is fiscal resident in a country where capital gains are not taxable. This person can withdraw the money without bothering about hiding anything. Then have this person give you the money either officially (if under the yearly limit set for donation / gift) or by cash. Or even better, sell something to this person on ebay, and receive officially the proceeds by wire (so long as the amount remains within the limits set by IRS for private selling as individual).