The official Pokémon website has announced the details for the Jirachi distribution. GameStop will be allowing players of Pokémon Diamond, Pearl, and Platinum to download Jirachi from February 27th, 2010 to March 13th, 2010.

Players can trade the Jirachi they receive into Pokémon HeartGold or SoulSilver and it will unlock a new special route inside the Pokéwalker called Night Sky's Edge. This is the only way this route can be accessed! Players may only receive one Jirachi per game cart.

Jirachi is a Level 5 Pokémon and it holds a Liechi Berry. Jirachi also knows Wish, Confusion, Rest and Draco Meteor.

Source: http://www.pokemon.com/us/special/jirachi