Update: The Pokemon Company has confirmed that mega evolutions will be coming to Pokemon X and Y, and has released a trailer detailing the feature. Applicable Pokemon will require a rare mega stone, however, and Nintendo will distribute a Torchic holding a Blazikenite mega stone over Wi-Fi during the game's launch.
Additionally, Pokemon X and Y will allow players to boost a Pokemon's stats with Super Training, which features a range of mini-games, or with Core Training, which allows Pokemon to get stronger without player input.
The information was first posted in Japanese comic CoroCoro, and includes scans of MegaMewtwo, MegaBlaziken, MegaLucario, MegaMawile, MegaAbsol, and MegaAmpharos.
Serebii's report says that Pokemon can only Mega Evolve when holding a Mega Stone, with the temporary form giving a temporary new appearance and boosts to statistics. The Mega evolutions are somehow connected to the game's Master Tower.
Pokemon X and Y will launch exclusively for Nintendo 3DS on October 12.