Source: http://www.pokemon.com/us/ex/mewtwo/get-mewtwo

Mewtwo will be distributed to English language versions of Pokémon Black and White via a WiFi event starting February 12th.

According to the announcement, Mewtwo will be know a move that it can't normally learn in the games. Although the move isn't specified, a similar event was held in Japan and the Mewtwo distributed there knew the moves Electro Ball, Shadow Ball, Aura Sphere, and Psystrike. It is currently not known when the event will end.