Obscuring the source won't help if the IRS knows "John Doe at least once controlled the Bitcoins at address foo". At that point, they'll assume you still control them unless you can prove otherwise.
Unless the money came from an illicit source (I don't think the source was stated, but making a ton of money from Bitcoin's rise is perfectly legitimate), cashing out would surely be perfectly legitimate as long as one paid the right taxes, which there would hardly be a reason not to.