For Poképlayers who truly, positively, absolutely must catch them all, Nintendo is introducing an online Pokémon storage system which charges an annual fee. The Pokémon Bank will store up to 3,000 pocket monsters, safe from harm and easily transferred between games.
Pokémon X and Y players will be able to store their fighters online in the Bank (the Cloud, as you kids would say), then download them back into either as they please. If you're an obsessive and plan to buy both versions, yes, this means you won't need two handhelds to swap a Pokémon from one to the other.
Bank will be a downloadable app for 3DS, accompanied by the Poké Transporter app. The Transporter can upload pocket monsters from earlier games Pokémon Black and White (and v2 of these) into the Bank, though it's only a one-way transfer and can't return any to B&W afterwards.
Nintendo hasn't revealed how much the annual fee will cost. Check the announcement for more. Pokémon X and Y are due to arrive for Nintendo 3DS on October 12.