I'm guessing a lot of people are waiting until after the 1st of the year to sell their coins in order to cash out and realize a profit. Cashing out after the 1st of the year allows them one whole year to find tax havens such as mortgage interest and Roth IRAs.
My understanding of tax law in the US is that, for forex (and bitcoin), you pay taxes on a marked to market basis. You calculate dollars out (or dollar value of bitcoins) minus dollars in, and pay taxes on that. It does not matter whether you "cash out" or not -- the tax is the same.