The official Pokémon website has announced new rule changes that will take effect at the end of the year for Pokémon Black and White online battling. The rule changes are expected to improve the competitive balance by making gameplay more fair for all. Below is the news from the official Pokémon website detailing the changes.

New rules for online battles will improve competitive balance for everyone!

Get ready for a better experience when battling online. Starting December 26th for Free Mode and December 27th for Rating Mode, Random Matchup battles played through Nintendo Wi-Fi Connection, as well as official Wi-Fi Competitions in the Global Battle Union, will have a couple of new regulations. First, as a reminder, all of the moves known by Pokémon in your party must have been learned by the following methods: by leveling up, by using TMs or HMs, as Egg Moves, via in-game events such as the Move Tutor, or through official Pokémon distributions. In addition, Dark Void joins the move Sky Drop as a prohibited move in Random Matchups and Wi-Fi Competitions.

Also, saved data or Pokémon that have been modified by cheat codes or game-altering devices are not allowed. Not only do they detract from the spirit of fair play, they can cause your data to become corrupted or unstable, or to disappear completely.

If you try to connect to Random Matchup and you have saved data or a Pokémon in your party that is against the rules, you will receive the message: "There was a problem with your Pokémon, and they could not participate." If you see this message, check your Pokémon party, including each Pokémon‘s moves and held items. Also, read over the battle rules to confirm that there are no conflicts.

These changes should make Nintendo WFC battles more fun for everyone. Plus, these rules will align online competitions with Play! Pokémon tournaments, ensuring that your team is ready to battle no matter where you decide to compete.

Thank you for your understanding, and have fun battling in the Global Battle Union!

Source: http://www.pokemon.com/us/news/pgl_rulechanges-2011-12-19/