Korean Arduino enthusiast 'Hounjini' has documented a method of two-way data communication between an Arduino and a Nintendo DS video game system. The hardware side of things could be a little tricky for the beginner, but it is possible. Furthermore there is also a library for the Arduino<>DS communication. Once it's working, you can retrieve touchpad position data and also read the DS buttons. For example:
If you decide to start experimenting with the your NDS, be sure to heed the warnings in the instructions. To get started you can find the instructions here. And we're on twitter, so follow us for news and product updates as well.