We just released the first revision of DS brut, an open source hardware prototyping platform for the Nintendo DS. An inexpensive cartridge for Slot-1 provides for basic UART, up to 6 GPIO lines, up to three ports for PWM, up to two ADCs and I²C bus - all accessible from within your homebrew NDS code.
The cartridges embedded microcontroller can be easily reprogrammed via its serial port. We are also releasing the LGPL-licensed firmware and an accompanying library for the Nintendo DS that handles communication with the device and hides all complexity behind Arduino-style function calls (think analogRead()) that help you get things done in a couple of lines code!
We'll be shipping out the first batch of cartridges sometime next week and looking forward for your creations with this tool!