At Game Developers Conference today, Nintendo project leader Masato Kuwahara revealed that DSi enhanced game carts are in the works. Essentially, these are carts that still work in a regular DS or DS Lite, but will take advantage of DSi-specific hardware features.
If no plans to pick up a DSi are on the table, don't freak out. Nintendo obviously does not want to splinter the market here, so I wouldn't expect the DSi-only features to go far beyond taking advantage of features such as the camera. These enhancements will likely be small perks for DSi owners - just like how certain Game Boy titles offered extra but non-essential content when inserted into a Game Boy Color.
Nintendo Plans "Nintendo DSi Enhanced" Game Cards With DSi-Only Features [Kotaku]