Even though the PS3 predecessor of Arkham City had a playable Joker for the challenge maps, the PC and 360 versions had absolutely nothing special. Nice, but Arkham City is going to have no exclusive content on any of the platforms it appears on.
Dax Ginn stated at Gamescom “We’re really treating gamers very equally regardless of which platform they want to play the game on. The plan at the moment is really to deliver the same game across all platforms.”, the only exclusives that will divide the users are pre-order skin DLC. A purely cosmetic thing that not everyone has a major interest in, and everyone that does will have a favorite skin for the Dark Knight. There are also some pre-orders that give access to Robin and some challenge maps.
Even if you don’t like the idea of pre-order bonuses, Rocksteady has announced they will be putting the DLC up for purchase after the game’s release for those that really want it.
Games with multiple platform releases will sometimes have exclusive content for multiple reasons, including liscenses, just for the sake of it, or as something they simply couldn’t have in the original release if the port comes later. An example is a cross-platform Soul Calibur game, which I believe had a version with Link, and a version with Darth Vader.
Basically, regardless of what version you get and when you get it, you are not going to be locked out of the DLC from the looks of it. Good news that.