According to Nintendo.gr, official sources state that Pokémon Black and White are hitting the European shelves in Spring 2011. What is more intriguing is that they named the two Legendary Pokémon which are going to be the mascots of each version: Reshiram [レシラム] for Pokémon Black and Zekrom [ゼクロム] for Pokémon White. They also mention about a new storyline, though nothing is clear about it currently.
Edit: It seems that the US release is also scheduled for Spring 2011.
Edit 2: Pokemon.com has a picture of the two legendaries! As many may have predicted they look like a holy beast and a Demon BUT the white-coloured Pokémon is the star of Pokémon Black and vice-versa!
Edit 3: The official Pokémon Series website, has a proper video of walking around in Hiun City.
Edit 4: Japanese Boxart has been revealed as well. It is also now known that the new Legendary Pokémon are Dragon-type. They may have a second type but it’s unknown at the moment.
Source: Nintendo