In serebii's main page, it is mentioned that a person went to an early screening of the Keldeo Movie & saw Genosect in the next movie trailer, stating it will air in 2013.
I guess several of us expected it but what do you expect?
"ExtremeSpeed Genesect & The Reawakening of Mewtwo"
Small Summary: "Genesect", a Pokemon from 300 million years ago. The mysterious "Red Genesect", different from regular Genesect, appears. Possessing super speed and boasting other strengths, it is a mythical Pokemon, but what sort of activity will it do in a big city, the symbol of modern civilization?" - Thanks to Sunnyshore for the Summary.
Spoiler:- December 2012 CoroCoro:
Pokemon Smash Movie Announcement
LINK: =====> 12/12/12- First Movie 16 Trailer!!!
2/12/2013- CoroCoro revealed a new title: "ExtremeSpeed Genesect & The Reawakening of Mewtwo".
2/15/2013- New Trailer
Spoiler:- New Poster with Sableye!: