As posted on Mamytwink.com (French site but video is in English) http://www.mamytwink.com/actualite/w...la-precommande.
They have linked a video from World of Warcraft's youtube site (video is unlisted so not "officially" released yet) about the pre-purchase and free level 90 upgrade. I would imagine it is coming very soon!
http://www.youtube.com/watch?v=TaBYjVBY_bM