Some says that you are able to trade Mega-stones from X and Y to ORAS but you will not be able to get ORAS Mega-stones into X and Y. There are also other people saying Mega-stones will not be able to trade across the game at all. Personally I think that ORAS will be the main game for Gen VI so it has access to all the Mega-stones. Game Freak leaders have already announced that they will not patch X and Y to include ORAS Megas.
http://www.nintendolife.com/news/201...ega_evolutions
http://www.gonintendo.com/s/239469-j...ideo-interview
Also another good informative interview