For those of you in the U.S. Gamestop will be distributing 3 legendary Pokemon to all 5th Gen games (i.e. Black, White, Black 2 and White 2).
You will be able to get a Shiny Dialga beginning August 19th and running until September 8th. From September 9th until September 29th you'll be able to get a Shiny Palkia. Finally, from September 30th to October 20th you'll be able to pick up a Shiny Giratina!
No word yet on whether this event will be happening outside the U.S. We'll be sure to let you know more details when they become available.
UPDATE: The distribution will also be available from "participating retailers" in the U.K (some GAME Facebook pages have announced they'll have the distribution). The dates are as follows.
UPDATE 2: Australia and New Zealand are getting the distributions at select EB Games and JB Hi-Fi stores for the same dates as the U.K.
Shiny Dialga: August 30th - September 12th
Shiny Palkia: September 13th - September 26th
Shiny Giratina: Septembr 27th - October 11th
Sources:
Pokemon.com
U.K. Pokemon.com
AUS Pokemon Black & White Site
vBulletin Message