It will be very tough to enforce for the consumer, but it will be just as hard for a business like Overstock (that needs to keep diligent records) to remain above board on the tax code if they are accepting bitcoins as payment. Not only that, but it makes accepting bitcoins a major liability for any business that does numerous transactions and doesn't want the IRS crawling through their books. This seems to be the US government saying, fuck bitcoins, without having to say fuck bitcoins. I would not be surprised if you see the few major company's that do accept bitcoins stop doing so in the coming weeks. I hope I'm wrong, but that's the way I see it.