On September 10th, a book called Pokemon Pia will go on sale in Japan.
This book will contain Pikachu earphones (neat, sorta!) as well as a password for you to enter in your Pokemon Global Link account to encounter one of the three Kanto starters (with different abilities) in the Pokemon Dream World (PDW) and sync back to your Black or White game save.
I plan on buying one, but you can only enter one password per account.... I think? (Can't find confirmation on that yet... actually, I really have no idea on this as there isn't any official word, I just heard it on other sites)
I have no idea how the passwords work. There may be millions of passwords that randomly generate one of the three starters upon being entered, or.... there maybe be only three universal passwords, one for each starter.
(it appears they are each individual, not set passwords)
If you feel ambitious and want to try collecting one or more of these for the site, the Amazon.jp link is above.
Also, if you find any more info about this event, feel free to post it here.
(See my thread about the Eeveelutions event for PGL/PDW for info regarding the PGL and backing up your save files.)