From August 27 to October 6, GameStop locations in the United States will be offering Keldeo for free to those who bring in their DS or 3DS with a copy of Pokémon Black or White. Drat! We'll have to leave the house! For Europeans, head over to Game or Gamestation between September 1 and October 11.
Interestingly, the Water / Fighting-type Pokémon gets a bit of an upgrade if you transfer it to either Pokémon Black Version 2 or White Version 2, once those games release in October. Doing so will give Keldeo the Secret Sword move, causing it to change into its Resolute Form.
The last time I played my copy of White, I was foolishly in the process of transferring all of my Pokémon from prior games so the latest iteration of the Pokédex could be filled out. That was ... an experience.