Pokémon Omega Ruby and Pokémon Alpha Sapphire announced

We now know how compatability will work between X/Y & ORAS:

Battles:

Online and wireless battles in the games remains exactly the same as before. While between Pokémon Omega Ruby & Alpha Sapphire games you can do and use anything, when you connect with players of Pokémon X & Y, there are several restrictions. Essentially, you cannot use any of the new Mega Evolutions, new Primal Reversions, new items, new moves and new abilities in battle against these players. The game will inform you as you select your team whether or not it can be used.

The Battle Spot changes this up somewhat. In Free Battle, Pokémon X & Y and Pokémon Omega Ruby & Alpha Sapphire run different matchmaking schemes so they will never encounter each other. In Rating Battle, they also run different schemes, with Pokémon Omega Ruby & Alpha Sapphire's scheme changing somewhat from the standard of Pokémon X & Y

• ΩR/αS League: Open only for players of Pokémon Omega Ruby & Alpha Sapphire. This league lets you only use Pokémon that were caught, hatched or distributed in Pokémon X, Y, Omega Ruby & Alpha Sapphire.
  
• XY League: Open only for players of Pokémon X & Y. This league lets you only use any Pokémon, including those that were transferred from Pokémon Black, White, Black 2 or White 2.
  

Trades:

Trading is the other one of the core aspects of Pokémon and also has several restrictions. Many of these restrictions will be akin to those in Pokémon X & Y where you couldn't trade Pokémon holding the Discount Coupon online. Like battling, you won't be able to trade Pokémon who hold any of the new Mega Stones over to Pokémon X & Y, but you can trade them fine between various copies of Pokémon Omega Ruby & Alpha Sapphire. You also cannot trade Pokémon who have the new moves that are introduced in the new game.

Currently, it is not clear as to how the special PSS features of Wonder Trade and the Global Trade Station will run, whether or not they run separate servers for the two games or if there will be restrictions.

Pokémon Bank:

Pokémon Bank is a piece of software that was introduced after Pokémon X & Y to store the various Pokémon obtained in the game. Pokémon Omega Ruby & Alpha Sapphire will contain full compatibility with Pokémon Bank at launch.

As Pokémon Bank removes the items that are on the Pokémon before putting them in the Bank, compatibility isn't much of an issue. However, it is not currently known how the software will handle the new moves, as to whether they'll upload and not be able to be moved to Pokémon X & Y later, or if they'll be blocked from uploading.