Corey Menscher, an ITP student, has designed a kick sensor which monitors his pregnant wife's belly, and generates a fetal tweet whenever the baby kicks.

The thing seems to have a bug:

Wow I'm being very active! I kicked Mommy 1766 times at 11:59AM on Thu, Dec 11!

Regardless its another interesting use of twitter. Implementation details here:

The Kickbee is a wearable device made of a stretchable band and embedded electronics and sensors. Piezo sensors are attached directly to the band, and transmit voltages when movement underneath is detected. An Arduino Mini transmits the signals to an accompanying Java application wirelessly via Bluetooth.

The Java application receives the sensor values and analyzes them. When a kick event is detected, a Twitter message is posted via the Twitter API.