A Jirachi is going to be distributed to Japanese Black, White, Black 2, and White 2 games from July 1st to July 31st in a tie-in with some episode of the anime in July.
We don't know if it's a wifi or local distribution yet, because it was only teased in Corocoro, but it's probably wifi.
More details when it's officially announced.
Update:
http://www.pokemon.co.jp/info/2013/06/130615_g01.html
It's a Nintendo Zone and Wi-Fi event.
Jirachi is level 50
Knows Healing Wish, Psychic, Swift, Meteor Mash (event move)
Cherish Ball, Wish Ribbon, Serene Grace, holding Micle Berry
OT is: デコロラ, 07013