Nintendo Europe has announced this morning that the release date for Pokemon Heart Gold and Soul Silver in the UK is 26th March.
In addition they have confirmed that the Pokewalker peripheral is also due for release in Europe (Americans should be getting it too, we would imagine) and they've even been kind enough to give some additional details on how the little device operates.
The Pokewalker has a pedometer in it so you can transport a single Pokemon into the device and take it on a walk with you.
The Pokemon in the Pokewalker will gain experience as you walk, making for an alternate means of training. Once you are finished walking the Pokemon, collected items and captured Pokemon can be transferred back to Pokemon SoulSilver and HeartGold.
If you know a friend or two also taking their Pokemon for a walk, you can use the infrared port on the device to get gifts from each other. Older Pokefans may be reminded of the Virtual Pokemon "Tamagotchi" from back in the late '90s, so this is essentially a game and a virtual pet in a box (but without the need to feed and clean 'em all thankfully!)