Actually, I'm pretty sure the IRS recently acknowledged that Bitcoin was considered "Property". That means they're going to tax it. They just don't know how to find it efficiently yet, so it looks like they're working on the good faith principal for now that you'll just report how much you make. Which is the beauty of crypto currency in the first place! People just need to make sure they are being diligent and not leaving an obvious paper trail about what they do with their coins.
Edit: Yeah, I was right. https://www.irs.gov/uac/newsroom/irs-virtual-currency-guidance
I don't speak legalese, but that sounds pretty clear that they consider any profits you make from crypto (well, bitcoin at least) is taxable as either income or gains, whatever the circumstance dictates.