Masato Kuwahara, DSi Hardware project leader, recently talked about specific DSi retail game cards.
Previous press releases and interviews have suggested that DSi-specific retail games will be available, and at GDC it's become official. "Enhanced game cards" will offer developers the opportunity to use some of the specific features - e.g. camera and SD card - in their games.
For example, you will use these features if you want to create textures from photographs taken with the DSi camera. The Inspiration Behind Nintendo DSi Development, that these types of "Nintendo DSi Enhanced" game cards are still in development.
Games can be played on a regular DS/Lite, but specific features will be locked to those with DSi. To avoid alienating a huge number of consumers, it's likely the functionality will be for extras, bonus features, but you never quite know..