The important question is *when* you report it.  I have made money in the Second Life virtual world for the last 6 years, and it has it's own virtual currency used internally.  But the owners of Second Life reserve the right to delete your account for any reason.  So my balances in virtual currency are not "in my hands" yet, they are just a data entry on their servers.  What I do is report cash out to US dollars at the point it reaches PayPal from Second Life.  At that point I have "constructive receipt" in the sense the IRS uses it, meaning I can spend the PayPal balance on other stuff, or deposit it in checking like I would a paycheck from a job.  So far, the IRS has not questioned my returns.

An issue with reporting accumulated bitcoin mining balances as income is what value to put on them, since that varies constantly.