Serebii,net has recently revealed that Venusaur, Charizard, and Blastoise will also be receiving Mega Forms in Pokemon X & Y. Here's a video of them in action.

It also appears that in order to activate Mega Evolutions, the player must have the bracelet shown in the video called the Mega Ring, which holds a stone called the Key Stone. The Key Stone in the Bracelet apparently resonates with the Pokemon's Mega Stone and triggers the evolution. I'll be sure to put up more info as it comes, so stay tuned.