Casai modified a bathroom scale to send his weight info to an Android app through Bluetooth. He documented his built, and released the firmware, as well as the mods he made to the open source Weight Chart app. The scale uses an ARM LPC2103 microcontroller and a serial Bluetooth module to communicate with a Android smart phone.
Smart Scale is a cheap ($75) and Open Source bathroom scale with wireless Bluetooth communications. It works just like a usual bathroom scale, where the user can use it and read the weight value on the LCD or the weight value is received automatically on user Android Smart Phone (which is sent by the scale wireless).