I just got my preordered copy of UM and the guys at the store were kind enough to send me a Marshadow Code too. Since I have no need for it, I'll give it to anyone who manages to guess my favorite Pokémon. Don't forget to leave an email adress with your guesses so I can send it to you. Keep in mind that this is an EU code and will thus not work in other regions.
Here's a hint: This Pokémon doesn't learn any special moves through leveling up.