Nintendo's Pokemon announcement today included the reveal of Mega Evolutions for Charizard, Blastoise, and Venusaur, but the big news was an announcement of a cloud-based storage solution for Generation V and VI Pokemon. The newly announced Pokemon Bank will let you store up to 3,000 Pokemon online in a cloud storage box.
This will, as Nintendo explained, allow you to transfer Pokemon to and from new generations of the game, starting with the Generation V to VI transfer. Transferring from Black & White (1 or 2) to the Bank (allowing them to be transported into X & Y) requires a separate Poke Transporter, which will presumably not be required in future generational shifts, and is not required to transfer from X & Y into the bank. The process was detailed in a diagram below.
The first month of the bank is free, so if you want to open it just to quickly swap your Black & White Pokemon into X & Y you are free to do so. After that Nintendo charges a fee of $5 per year for the server costs associated with the hosting, which dedicated Pokemon fans will likely find pretty reasonable for storage space for 3,000 Pokemon and insurance to not lose their favorites in a new generation.