I have spend the past week to get into homebrew NintendoDS development, went pretty well so far. The development environment in the form of devkitPro is solid and very easy to install, just untar, set two environment variables and you can start compiling NintendoDS software, some nice examples are provided as well.
Testing the NintendoDS software is a littel bit more tricky, so far I haven't found any NintendoDS emulator for GNU/Linux, some of the Windows ones however do work well under Wine. DeSmuME works the best for me closly followed by no$gba, the other in form of iDeas, ensata, Dualis and DSemu either failed to run under Wine or more likly simply failed to emulate the NintendoDS in a form that would make them usefull for testing (ie. either far to slow, completly screwed up display and such).
On development on the NintendoDS itself, its actually very easy, load a bit of graphics in some memory location here, set some bit flags there and more sooner then later you end up with something that looks already like half a game. One throuble however is the touchscreen, while easy to get basic coordinates, its extremly imprecise in the center, meaning your cursor will jump around like crazy unless you push quite hard on the touchscreen, some interpolation might help to remove those jitters, however havn't tested them yet.
Anyway, here two videos which show what I have done so far: