It looks like there is quite a lot of giveaways leading up to the release of Black/White.
January 3rd to February 9th will see Gamestop giving away the three
Legendary Dogs, each occupying a different week during the promotion.
January 30th to February 5th will see Toys R Us giving away
Ash's Pikachu.
And as mentioned above, you can get
Celebi from GameStop between February 27th and March 7th. It also notes that trading Celebi up to Black/White will give you access to the new Pokemon,
Zorua.
Oddly enough, I don't see anything listed on GameStop's website about these promotions. Perhaps the Celebi one is too far off, but the Legendary Dogs giveaway begins tomorrow! I'll have to call up to my local GameStop and check, I suppose. As much as I despise going into that place, it wouldn't be too bad for the sake of giving Entei some company on my game.