http://www.serebii.net/games/currentevents.shtml#japan-wifi-upcoming
Japan will soon get a Mystery Gift for Jirachi. This is the fourth Wi-Fi event so far, with the first three being the Rotom Secret Key, Oak's Letter (Shaymin), and Member's Card (Darkrai).
It is the first Wi-Fi Mystery Gift to:
- Support Diamond/Pearl (the above three are Platinum-only)
- Give you a Pokemon directly instead of letting you battle it
- Have some special event in a future game (Shiny Pichu, Regigigas, and Arceus are all Japan-only movie events I believe)
- Be related to Pokemon outside of the 4th gen set of #387-493
This downloadable Jirachi also unlocks some sort of special event in HG/SS that the Colosseum Bonus Disc Jirachis will presumably not unlock.
I happen to have a Japanese copy of Diamond, so I'll be getting this in three days on June 19. This Mystery Gift's status for US games is unknown, but it's a somewhat safe bet to guess that we will get this same Wi-Fi event within the next year, likely closer to 6 months.
As of this post edit, the Jirachi Mystery Gift is now available on Japanese games.