First off, is the development of DS software without official Nintendo licensing illegal? 'Cause I've been looking at some homebrew games people have developed and I know certain companies sell blank discs for use with the DS and GBA. Secondly, is anyone currently developing on the DS or does anyone have any ideas? I'd like to hear what others have to say about the development process on this system. And lastly, does anyone have any idea how well the mic on the DS works? Is it just enough to pick out some blochy sounds, or could one utilize a nice Record feature?

Commercially selling a game without Nintendo's approval is definitely illegal. Modding and developing on your own is probably a pretty gray area. Standard gamedev advice: if you're planning anything big, ask an actual lawyer, not the internet. In general, even if it was illegal, retail companies don't generally go after free, non-commercial projects unless they get too popular and become a threat to their intellectual property. If all you want to do is make a little game in your spare time and share it with a few friends, you probably don't need to worry about the FBI breaking your door down.

I don't know specifics about the mic, but I've heard rumors that it's pretty low quality, which is why half the games just have you blow into it or yell something rather than trying to recognize actual voice.