A very ambitious homebrewer who goes by the name KeithE has developed a motion-sensing card for the Nintendo DS. It consists of a 3-axis accelerometer and a single axis gyroscope. The card works on both, the DS and the DS Lite.

Here's a short blurb on the creator's website about how it works:

The DS Motion Card contains a Kionix tri-axis accelerometer and single-axis gyroscope that communicate with the DS using SPI (Serial Peripheral Interface) on the DS card.

The accelerometer measures acceleration in all directions (both gravitational acceleration and acceleration caused by a change in velocity). The gyroscope measures angular rotation rate (speed of rotation) around an axis perpendicular to the bottom screen of the DS.

Custom software turns the sensors on, reads the accelerations and angular rate, and controls the display based on the sensor readings. Functions to communicate with the sensors have already been written and are simple to integrate into any existing or new projects.

For more information and source code, download the DS Motion Card QuickStart Guide (zip).

I must say, this sounds rather intriguing. Motion-sensing on the DS would definitely be the next logical step for future upgrades. Are you seeing this, Nintendo? For those of you who are interested, you can find out more about this device at the creator's website, including some interesting uses he has come up with for the card and where to buy it.