First we know that if pre-ordered from Best Buy the person will receive a combo pack. Though not much is know we do know, that it will most likely give a new bundle of move sets. As if it didn't look good already:
Next, when Arkham Asylum, if pre-ordered from Gamestop, the player would receive a Scarecrow map. Now with Arkham City, the player will recieve a map called the "Jokers Carnival" which sounds fun enough already. This will lead to more exploration, and even more fun playable area's to explore in the vast city.
But I am very sorry to say these are exclusive to the US.
Though this is all we know, we do know there is more information to come regarding the bonuses that will be included for Amazon and Walmart. They shall most likely have very interesting features.
Batman: Arkham City coming out October 18, 2011.