Possibly the best fansite I've seen, Arkhamcity.co.uk captured the image below for anyone considering where to leave $5 behind. Best Buy's online product listing for Arkham City listed a playable Robin skin as a bonus. The skin is reportedly only available for use in the game's challenge maps, but it's playable Robin nonetheless.
If you haven't already put money down on Arkham City, I doubt you'll find a better incentive. Regardless, I hope that WB Games follow the trend they set with Mortal Kombat and offer all preorder DLC as a bundle after release.