The official Pokemon website announced that Pikachu Colored Pichu will be distributed at GameStop stores from January 30th, 2010 to February 14th, 2010. Players can bring in a copy of Pokémon Diamond, Pearl or Platinum Version and receive the special alternate colored Pichu.

When Pokémon HeartGold and SoulSilver are released, player will then be able to transfer the Pikachu Colored Pichu from their Pokémon Diamond, Pearl or Platinum Version. Players can then take Pikachu Colored Pichu to Ilex Forest (ウバメのもり) where Spiky-Eared Pichu (ギザみみピチュー) will join their team.

Source: http://www.pokemon.com/us/news/vg_pichu_promo-2010-01-12/