I was wondering the same thing. It looks like it is. Its sales ranking is No. 1 for Xbox strategy games, and 59 overall in games. Maybe they pre-sold so many that their stock is tapped out. Maybe they'll get more and reactivate pre-orders.
I saw last night that Saints Row the Third also wasn't available for pre-order anymore. But within a few minutes, it was back up.
I think a similar thing happened with Batman: Arkham City, or some other big-name game.
While Amazon has a $10 credit toward a future purchase along with the pre-order, if you don't care much for that, you could always get the game at brick-and-mortar stores. While the game's popular, I imagine there will be plenty at the various stores that carry games.