Pokemon who get their own entry in Wikipedia are pretty rare, there being 646 Pokemon available at this point, so Mewtwo is pretty important in the Pokemon mythos. Nintendo says about the impending Mewtwo event:
The legend of the Genetic Pokémon Mewtwo has grown ever since it debuted in the Pokémon Red Version and Pokémon Blue Version games. The Psychic-type Pokémon has always been considered one of the most powerful Pokémon available, and also very tough to catch.
You’ll have a new opportunity to get the Legendary Mewtwo once again! Starting on February 12th, you can receive Mewtwo for your Pokémon Black Version or Pokémon White Version game. Take part in a limited-time distribution event to get this amazing Pokémon. This Mewtwo is incredibly strong and fast, and it even has an exclusive move that Mewtwo cannot normally learn!
However, while Nintendo is being coy, Siliconera is reporting that they have already done this same download event in Japan and they are unlikely to change things up for the distribution in North America.
So if you have Pokemon Black or White, be ready come February 12th for this Nintendo WiFi special offer. I suspect the instructions will be similar to the Victini event they had at the end of December.