To show appreciation to customers/players for their patronage in 2011, Pokemon Centers in Japan will be distributing a Shaymin Mystery Gift to Black and White games from December 19th to January 5th.
Kind of a nice choice, since Shaymin is the Gratitude Pokemon.
If you show the Shaymin, changed into Sky Forme, to staff at one of the stores between Jan 1 and 15, they'll give you a special Shaymin Thank You card.
http://www.pokemon.co.jp/info/2011/11/111125_p04.html?rss=1005
More details when they get announced.
Edit: I've changed the thread title and references to this Pokemon in the Event Collection and Gallery from "Thank You Shaymin" to "Gratitude Shaymin."
The official website was referring to this as "「今年も1年ありがとう!」という感謝の気持ちを込めて" ... ie, saying "as a way of saying 'Thank you for this year as well,' with a feeling of gratitude" ... (here's a Shaymin present for your BW games). I wasn't crazy about "Thank You Shaymin" as a title, but that seemed like the best way to sum it up at the time.... anyway, the Wonder Card has now been released, and its title is かんしゃのきもち!シェイミプレゼント... and かんしゃのきもち is 感謝の気持, ie "Feeling of Gratitude." SO, I decided to change our title to "Gratitude Shaymin."