The Moto X was released today along with the Moto Maker customization tool. Moto Maker doesn't yet offer the option for a woodgrain back plate for the device, but we know that will be happening eventually. However, if you take a look at the Motorola website source code, it seems that the special back plate could cost you extra.
This is definitely not confirmation, and the website code could just have a random number in there as a placeholder. But, if you were to dig into the site code, it lists all of the various customization options for the Moto X, including the wooden back plate which is not yet available. If the source code is to be believed, getting a woodgrain back plate could cost you an extra $50.
It definitely wouldn't be a surprise if Motorola were to charge a premium for the wood back. The real question is: would you pay $50 extra for it?