Like Pokmon Platinum, Heart Gold & Soul Silver have got special details implemented within the game if you send certain Pokmon over.. In Platinum, this allowed for new forms and capturing of certain Pokmon. However, Heart Gold & Soul Silver have got a slightly different take on this.
If you send over the special Pikachu Coloured Pichu obtained from the special 12th movie event given away from June 19th to July 18th, a special event will activate. This event takes place in Ilex Forest. When you go by Celebi's Shrine in Ilex Forest when you have the Pikachu Coloured Pichu in your party, a Notched-Ear Pichu will appear. The further details of this are not yet known. We do not know if it's just a story piece or if it allows access to other Pokmon. We'll bring you more on this as soon as we can.