While the DSi plays the same games/cards as the DS, Nintendo, somewhat predictably, has announced plans to release "DSi Enhanced" games.
These games would still be cross-compatible, playable on both the DS and DSi. But these games would take advantage of a few special DSi functions not available to DS players. One example given was the ability to take a picture on the DSi and map it as a texture into the game (face mapping comes to mind.)
Sounds fine to us, though only time will tell whether or not DSi "Enhanced" ever becomes DSi "Required." [Kotaku]