This is Rittai Kakushi e Attakoreda, a DSiWare game that makes brilliant use of the Nintendo DSi's built-in cameras to control an in-game camera that lets the player explore a 3D diorama to find hidden objects.
The downloadable game, right now for Japan only, comes courtesy of Wario Land: Shake It! developer Good-Feel. As one can see in the preview video above, players must locate hidden objects and form letter shapes by tilting the DSi, and, in turn, the game world. Movement is tracked by processing input from the handheld's dual cameras.
Maybe not the killer app that will get you to upgrade from your DS Lite, but perhaps the coolest use of the DSi's unique features to date.