Does Mists of Pandaria include any credit for gameplay time on a WoW subscription?
It's only a rumor. Though each vendor may give different pre-order bonuses for ordering from them (e.g., Amazon offers you a key to gain access to the MoP beta), Blizzard isn't actually offering any particular incentive themselves.
No. Mists of Pandaria does not include any "game time". No prior expansion (BC, WotLK, Cata) included "game time" with purchase.
You do get "game time" with the purchase of the base game, or with purchases that include the base game (such as Battle Chest offers).
No. WoW expansions do not come with game time.