If you pre-order Theatrhythm (which you're going to do, right?) and you're from the US, you'll receive a pretty cool gift. As well as a unique Theatrhythm Final Fantasy stylus, you'll get a set of stickers with which to customize your stylus, your 3DS and anything else you want to deface. Huzzah!
I have to admit, I love the art style for Theatrhythm and I think it says something about Japan that characters from quite adult, often dark games can be redesigned in a different style and look so adorable. From Cloud, Squall and Lightning to classic series staples like the Chocobo or Moogle, it's a pre-order gift that may make your gaming better based on sheer "awwww" alone.
As far as pre-order bonuses go, a stylus isn't all that big a deal. Still, if you're buying the game anyway (again, why wouldn't you?), it's pretty cool. Here's hoping that we in Europe get something similar.