According to several Pokemon websites they have translated a text on a highly respected Japanese website which gives details on the game Pokemon Colosseum.

Apparently in the game you will be able to enter a 3D field and encounter some Pokemon, including old ones not available in the Game Boy Advance Pokemon Ruby and Sapphire.

These Pokemon will also be downloadable onto the Game Boy Advance games. It is unknown how many 'hidden' Pokemon will be in the game at present.