Some links to various development sites and useful resources: Nocash gbatek docs Excellent low level guide to the GBA and DS with extensive comparison of GBA and DS with one another.
GBAdev One of the major hubs for DS and GBA homebrew coding and discussion.
Devkitpro A popular set of tools and libraries for GBA and DS development amongst other things, devkitarm is contained within it.
Pin Eight Some useful coding suggestions (see VRAM management especially)
Drunken coders tutorials Some decent tutorials.
DStech wiki More low level infomation.