There are different pre-order bonuses depending on where you pre-order it from. Check the official site for more details.
In other news, the demo rocks. Of the 5 playable characters, I like Terra the best, but I was really impressed by how fun and different each of the characters were. While starting out I kept having some small annoyances about a couple of the characters, but when I finally understood them all, I really had none that I disliked playing, or that played too similarly to the others (and the villains you face also seem unique). It is giving me very positive thoughts for the final game, especially since I know how much is not in the demo (more characters, more abilities, level up system, equipment system, story mode, more levels, more music, alternate costumes, etcetera).