actually, audiosurf for NDS shouldn't be that impossible.
the main problem is processing the mp3 file.
which can simply be fixed by doing the following:
convert the mp3 to a more lightweight audio type (this is possible without too much quality loss)
and using PC software to do design the tracks.
example: you put a few mp3 files in a program on your PC.
for each mp3 it outputs 2 files: a .trk file containing the track design, and a .wav(or other audio type) containing the audio.
you then load the .trk and .wav onto a microSD and then insert it in either
A: a DS flashcart next to the .nds file(if it's taking the cheaper homebrew way)
B: a slot-2(gba-slot) card specially made for audiosurf(the way more expensive, but 100% legal and proffesional way)
using way A someone already made a version of amplitude for the DS(originally a PS1 game somewhat similar to audiosurf)
graphics would off course have to be simplified(removal of the scenery objects!)
best would be with 3 modes.
1:mono
2:pointman or pusher
3:double vision(muliplayer)