Lenovo itself offered something similar for prior Windows tablets, and now, Microsoft will do similar for its Surface Pro, Surface 2, and Surface Pro 2 tablets. According to the product page, the accessory is set to extend one's Surface battery life by up to 70%. The Power Cover weighs 0.55 pounds and measures 9.75mm thick, and sadly, do not have backlit keys.
They'll ship initially in the United States, Canada, and Japan in around a week, but the $199 asking price will likely spook some prospective buyers. You'll need to be a pretty serious road warrior to justify spending that sum for a bit of extra battery life, but those who frequently find themselves away from a power outlet may appreciate the option.