Gamestop has let slip that they will be holding an event to distribute the Pikachu-Colored Pichu in US stores between January 30th and February 14. The Pikachu-Colored Pichu (which is a fancy way to say they're handing out a shiny Pichu) can be traded to Pokemon HeartGold and SoulSilver in order to unlock an event in the Ilex Forest that allows the trainer to add the Spiky-Eared Pichu to their party.
Gamestop says that all you need to do is make sure you've got the Pokedex in the game you want the Pikachu-Colored Pichu on.
The Pikachu-Colored Pichu, assuming it's the same as its Japanese counterpart, will be male, level 30, holding an Everstone, with the moves Charge, Volt Tackle, Endure, and Endeavor.
The Spiky-Eared Pichu, again, assuming the same as its Japanese counterpart, is female, level 30, holding a Zap Plate, with the moves Volt Tackle, Helping Hand, Pain Split, and Swagger. Note that the Spiky-Eared Pichu cannot evolve or be traded.
On a related note, the MSRP for Pokemon HeartGold and SoulSilver has been raised to US $39.99, up from $34.99. Keep that in mind when you preorder.