It is interesting to see what sensors are available on the mobile handset that Edigital used for the Nunchi demonstration at the SHM:

Accelerometer : Detects motion of the handset. You could use this to detect shaking the handset, a person waking, etc.

Magnetometer: Basically a compass.

Proximity Sensor: This detects when an object gets close to the face of the device. Mostly used to detect when a person puts the phone up to the ear, so that the display touch features can be disabled.

Ambient Light sensor : Detects how much ambient light there is. Normally this is used to adjust screen brightness, but Edigital used this to detect what kind of environment the user was in. It was quite clever.

GPS: Used to determine location.

Sound transducer : Normally used as a microphone, but could be used to detect ambient noise levels.