The first WiFi event in Pokemon Heart Gold and Soul Silver allows you a new zone to walk through on your PokeWalker pedometer. You can stroll through Yellow Forest filled with Pikachu.
"Many PIKACHU live in harmony in this forest. Walk through here frequently, and try to find one!"
The Pikachus found here are holding berries and range in level from 10-15. Apparently the level 15 pikachus have the attacks Fly, Thunder, Growl and Tail Whip. Level 14 know Surf.
There are also items you can get, such as:
You can download the event at Toys R Us locations in the US and Europe. Choose wisely, for you can only receive 3 mystery gifts per game.
The next WiFi event is the 10th Anniversary Mew, and the release dates have yet to be announced.
After that, the third event gives you the item Enigma Stone which allows you access to Latias and Latios when you take the item to Pewter City Museum.