Parasytic over at the official Nintendo forums has created a video of what he claims to be unreleased maps for Mario Kart DS.  Here is what he had to say:

“I guess a LITTLE explanation is required for this...
There are several courses in Mario Kart DS that are unfinished and completely unavailable (beta). They can be played with a little bit of hacking, but not at all perfectly. The textures are missing, so the courses are displayed almost pure white. The only thing that can help guide you along is the minimal shading (or following the CPU racers).

Below is a video I captured to show the Mario Circuit (GCN) course as it appears in Mario Kart DS. Sorry about the super-white visuals, but it's the best I can do without any textures! “

Download: [Mario Kart DS Unreleased Tracks Video]