Famitsu reports that, according to Satoru Iwata, Wii and DS will share a relationship similar to the one between GameCube and GBA. For select Wii titles, the DS will be used as a bonus touch screen controller. In addition, Wii will potentially expand the playing field for certain DS games and will serve as a hub for downloadable DS demos via WiiConnect24.
Pokémon Battle Revolution will be the first Wii game to feature DS connectivity. Owners of a DS and Pokémon Diamond/Pearl will be able to transfer their critters to the Wii's Battle Revolution, and vice versa. Granted, this is a fine bonus, but we'd like to see more innovative uses of the Wii-DS hook up for future titles.