A couple of friends have emailed to ask if there are any good books that would help in getting a leg up on deving for the DS. Well naturally there are no specific books on the topic, but given that the DS runs on the ARM9 (and the ARM7), it never hurts to get comfortable with the ARM family of processors and RISC computing in general. To help you gain that comfort I suggest you cozy up with a few books, they are the most authoritative references on ARM that I'm aware of.
As the tagline to this blog professes I will be blogging on the subject of RISC assembly in the very near future. If you're really eager to begin working your ARM chops, the Gameboy Advance runs on the ARM7tdmi and a good deal of its concepts and inner-workings will ready you for the DS when it becomes brewable.