Frankly, I don't think it's a good idea. Developers of closed architectures like Nintendo DS tend to frown on unauthorized development for their systems. If the Nintendo DS is restricted in a similar fashion to the Wii, you would also run into the situation where a large number of users are either unwilling or unable to modify their system to use your software.
As a general rule, open source software and closed architectures don't mix. I remember reading somewhere that Sony even removed the ability to run Linux from their later PS3's.
The mobile phone idea is a good one, but be careful, there may be issues with unauthorized development there as well.
http://en.wikipedia.org/wiki/Closed_architecture