The Mewtwo will be available for download via WiFi from the 12th of February, 2012.
By speculation, the Mewtwo is probably based off the First Anniversary Mewtwo distributed in Japan from September to October last year.
Should this be the case, the Mewtwo will arrive at Level 70 in a Cherish Ball. It has Pressure as an ability and be holding a King's Rock. Its moveset will be Psystrike, Shadow Ball, Aura Sphere, and Electro Ball.
No end date has been announced yet.
Update from Guested (1/6/12): It ends the 26th of February or the 6th of March.
Update from Guested (2/27/2012): Turns out the end-date was the 26th.
Update (26/1/12): It has been confirmed that the Mewtwo is in fact based off the Japanese First Anniversary Mewtwo.
Source(s):
Pokémon Official Site (USA) Announcement
Pokémon Official Site News story
Here is my original post from PokemonGTS.com.