The official Pokémon HeartGold and SoulSilver website has announced that the Yellow Forest Pokéwalker Course will be available from April 1st, 2010 to May 5th, 2010. Originally the press release stated the course would end on April 30th, 2010, but that was later changed. Below is portions of the press release from the official site.

The aptly named Yellow Forest is a Pokéwalker Route that is full of Pikachu! While you're exploring the Yellow Forest, you might even come across Pikachu that have special moves. The only way you can get the Yellow Forest Route is via a special distribution!

Learn how to receive the Yellow Forest Route

1. Select "MYSTERY GIFT" on the Main Menu. (Note: You must have obtained the Pokédex in your game to select "MYSTERY GIFT.")
2. Select "RECEIVE GIFT"
3. After you select "RECEIVE GIFT," select "GET VIA NINTENDO WFC" and choose "YES."
4. Watch as you receive the Yellow Forest Route Map!
5. Go to any Poké Mart and talk to the deliveryman in green standing in front of the counter. He will give you the Yellow Forest Route Map.
6. The final step is to save your game. This is very important, so don't forget!
7. Select "CONNECT TO POKEéWALKER" from the Main Menu
8. Select a Pokémon from your PC Box to send to the Pokéwalker (You must have a Pokéwalker in your PC Box)
9. Select the Yellow Forest Route Map.
10. Connect to the Pokéwalker and now you are ready to stroll!

Source: http://www.pokemongoldsilver.com/us/#/news/yellow-forest