If the pattern from the last Generation holds true, the Mythical Pokémon (it's so nice to finally have an official term for hidden legendaries) for this Generation will be revealed (in Japan) on February 14th of the year their movies come out. Keldeo will probably be revealed 2/14/2012, Meloetta on 2/14/2013, and Genesect on 2/14/2014. I've never been able to figure out the significance of revealing the Mythical Pokémon on Valentine's Day, but maybe they just figured that's enough time to reveal it and have products, etc. of the new Pokémon ready to advertise.
Leaks of the Mythical Pokémon in Coro Coro Magazine will most likely occur days before the 14th.