When I see somebody pull out a digital guitar tuner thats the same size as a DS lite, I think it is benefitial. Also digital tuners that can pick up chords etc. (chromatic) are about $25 and up.

There are a few homebrew apps that use the mic.
http://www.double.co.nz/nintendo_ds/ has a tutorial on programming mic recordings.

What I envision would be controls via the touch pad and buttons, with the top screen showing a typical 1/4 circlie radial meter that changes ranges depending on the sound picked up. (hands free)

Here is an example of a basic chromatic tuners functions and look:
http://www.axemusic.com/product.asp?...=2788&PT_ID=50