The official website for HeartGold and SoulSilver has announced some downloadable content for the game. The Yellow Forest route will be made available for download. According to the website, the route will be full of pikachu including some with special moves. The route will be available on the Pokewalker.
So how do you get this route? Between April 2 and April 30, it'll be available by using Mystery Gift on Nintendo Wi-Fi Connect. Just follow the instructions on the page linked above. According Official Nintendo Magazine, it'll be available in both America and the UK at the same time.
So, what special moves to you think these pikachu will have? I'd imagine fly since they have a picture of flying pikachu in the background. If my guess is right, the pikachu on the left in the foreground is sleepy pikachu with the yawn attack. Maybe surfing pikachu like in Pokemon Yellow?