Anyone who doesn't have the special event Shiny Pichu on their Diamond, Pearl, or Platinum game yet and lives in North America, Europe, or Australia will now be able to get one via Wi-Fi. This is the first time a Pokémon is being sent directly to Pokémon main series games outside of Japan. The event is currently active and will remain so until March 25th. When traded to HeartGold and SoulSilver, this Pichu will allow you to unlock the Spiky-Eared Pichu.
So what does this mean for Jirachi? Well, according to a disclaimer at pokemon.com that event may be distributed over Wi-Fi as well. We'll be sure to bring you the news if it is.