For those of you who don't know yet, the Pokéwalker will come with every copy of Heart Gold or Soul Silver. It acts as a pedometer and counts your steps. You can then level up Pokémon, and do other activities as well (see the video below) Here are what fellow forum members think about the Pokéwalker!

"I'll walk with it to school proudly! Or just in my pocket..." - MBlaze "I'll walk around with it! In my pocket. I actually think it seems really cool though~ XD" - Fuzia "I too shall proudly wear it!" - Griffinheart "I'll have it in me Pocket while I am at work. I am bound to rack up the steps that way." - DXRenagade

"It has been tested, and shaking it for a minute will only earn you about 6 steps. How it can tell you're not walking, I'll never know." - Mewtoo22 "I will have it in my pocket as I go about school. Easy levels!" - Jojo "the buttons get pushed when its in your pocket or maybe its just that I wear tight pants..." - Cladstn_Vampire

 

Here is an article Kommeo posted from the GameStop website:

The brand-new Pokéwalker brings a dimension to Pokémon games that you’ve never seen before. The Pokéwalker is a special pedometer that you carry with you that lets you take Pokémon fun wherever you go! You can transfer a Pokémon onto the Pokéwalker accessory using an infrared connection between the Pokéwalker and your Game Card. Then as you Stroll around with your Pokémon, it’ll earn Experience Points to gain a level. And you can even find items and special Pokémon while you’re on the go! The Pokéwalker is small enough that you can keep it in your pocket and take wherever you go. It also has a clip to hook it on your belt, and a hole for attaching a strap. It’s never been easier to take Pokémon with you!

The Pokéwalker counts the number of steps you take as you walk around. The more steps you take, the more Watts you earn, which you can spend to find items and Pokémon!

When you transfer a Pokémon from your game into your Pokéwalker, you can select which Route you’d like to take your Pokémon along. Depending on which Route you take (such as in a grassland or by the sea), you’ll encounter different wild Pokémon and find remarkable items. When you first begin your journeys with the Pokéwalker, the list of Routes you can select from is short. But, the more you take a Stroll with your Pokémon, the more Routes will appear and the more Pokémon and items you’ll be able to get!

Catch Wild Pokémon!
After you’ve collected some Watts by going on a Stroll with your Pokémon, spend some of them to use the Poké Radar to catch wild Pokémon! If you see tall grass rustling after you use the Poké Radar, you know you’ve found a Pokémon. You have three options in the battle. You can attack, evade, or catch the wild Pokémon. By attacking, you can wear down the wild Pokémon to make it easier to catch. If you choose to evade when the wild Pokémon attacks, you’ll perform a counterattack! And when you think the timing is right, throw a Poké Ball to catch the Wild Pokémon. You can keep up to three caught Pokémon in your Pokéwalker at a time. If you catch a fourth Pokémon, you must choose one of them to set free.

Find Items!
Use the Pokéwalker’s Dowsing Machine to help find items. Like the Poké Radar, you use the Watts you earned while walking to use the Dowsing Machine. Some of the items you can find are quite uncommon! When you use the Dowsing Machine, you’ll see six patches of tall grass, and one of them has a hidden item in it. You have only two chances to discover the item! Some of the items you can get are really hard to find and very valuable, so keep hunting!

Time to Transfer!
Once you’ve found Pokémon and items using your Pokéwalker, you can send them back to your Pokémon game. You can only keep up to three Pokémon and three items in your Pokéwalker at a time, so be sure to send them to your game when your inventory fills up! Remember, if you catch a fourth Pokémon in your Pokéwalker, you’ll have to swap out one of the three Pokémon you’ve already caught.

The Pokéwalker Journal
After your Pokémon has spent some time in your Pokéwalker, you can get a recap in the Pokéwalker Journal. The journal tells of all the places the Pokémon has been and the things it has found while traveling inside your Pokéwalker! For example, if you catch a particular Pokémon while out on a Stroll, your Pokéwalker Journal will make note of it. It will also keep track of how your Pokémon has behaved while inside your Pokéwalker!

Pokéwalkers Communicate with Each Other!
The Pokéwalkers don’t communicate only with Game Cards—they can also communicate with each other! When you connect Pokéwalkers with a friend, you’ll both receive an item as a gift! You can connect two Pokéwalkers via infrared signal by pointing them at each other. When you do so, both players will receive an item! You can receive up to ten items by connecting to your friends, but you can get only one gift per person, per day.