A:
The 3DS will be backwards compatible with all DS and DSi games.
Those games will not be able to display imagery in 3D despite
being used on the 3DS. However, some games, most notably Poke'mon
Black & White, will gain some enhanced functionality when played
on the new system. These cases should be considered extremely rare,
however.
We finally have a confirmation on the rumored Pokémon Black/White enhancements when played on the 3DS. Director Junichi Masuda said there's nothing like that available.
"Other than just being able to play the game normally, there's nothing special if you use a 3DS."