The official Pokémon HeartGold and SoulSilver site has announced a Wi-Fi event to allow people to download Jirachi to unlock the Night Sky's Edge Pokéwalker Route. The event will be available from June 26th, 2010 to July 16th, 2010. Below is the news from the official website.

Your chance to catch the elusive Jirachi is approaching! This Steel and Psychic-type Pokémon isn't available during normal gameplay, so don't miss out on this unique opportunity. The Jirachi you'll receive in this distribution event is special in a number of ways. First, the Level 5 Pokémon know the Draco Meteor attack, a powerful blow that also lowers Jirachi's Sp. Atack. Jirachi is holding a Liechi Berry, which raises Jirachi's Attack when its HP is low. Jirachi also wears a Classic Ribbon, an item exclusive to Pokémon received in events. And, as you'll see when you unleash Jirachi in battle, the wondrous Pokémon is stored inside a Cherish Ball, which is representative of how you got Jirachi. Receive Jirachi via Nintendo Wi-Fi Connection from June 26th to July 16th.

Transfer Jirachi!

Jirachi has at least one more secret left to reveal! If you received Jirachi in your copy of Pokémon Diamond, Pearl or Platinum Version, transfer the Pokémon into your copy of Pokémon HeartGold or SoulSilver Version to unlock a new Route for your Pokéwalker! If you received Jirachi directly into your copy of Pokémon HeartGold or SoulSilver Version, the Route will be available right away. This Route is called Night Sky's Edge, and it's not available through normal gameplay!

How to receive Jirachi

• You must have an open slot in your party
• You need to have two or fewer Wonder Cards
• Jirachi may only be received once per Game Card
• You must have a Nintendo DS and a copy of Pokémon Diamond, Pearl, Platinum, HeartGold or SoulSilver Version
• You must have the Pokédex in your game to claim Jirachi

Source: http://www.pokemongoldsilver.com/uk/#/news/jirachi/