They posted a photo of them on Facebook with the message, quote, "Coming Soon: The Sweater That Should Not Be. It's getting colder, holiday parties are about three months away and we want you to be ready for it. Metallica says that the sweaters will be exclusively available through their online store but there's no word when they'll be out.
(Here's the Facebook link to the sample photos. You have to look close to see the Metallica logos.)