If you guys dont already have, Id suggest getting some of the game development devices for the NDS. M3 Adapter/Simply is a good one, they can do much more than just play games on your NDS. You can listen to music, watch movies and turn your NDS into a PDA and much much more. There's even homebrew applications for email and irc if you are into chatting online.

Yeah that adds a lot more to the device, it turns into a PSP pretty much, except with the touch screen, which the PSP lacks. However, the adapter isn't too cheap at the moment, so prepare to pay ~$100 for it + the price of a DS to get full functionality out of your DS. There's some pretty good homebrew out for it right now, as well as emulators. And I think NDS rom backups work on it, if I'm not mistaken.