From GameBrew - a wiki dedicated to Video Game Homebrew.
Environment-Mapping Techdemo is one of the packages released by Payk to help people creating 3D homebrew.
Libnds does come along with some examples. In the 3d section is a sample called 'Env_mapping'. It shows how to use hardware sphere-mapping with a static texture. It is not a real 'Environment mapping', since the DS is basically to slow to distort an image and take it as texture on the fly. Sgstair helped to create a function to distort an image, and a table has been made of it by Payk.
It allows everyone to make real-time reflection on NDS.
Doesn't work on emulators.