Inside a Polar Wearlink chest strap fitness heart rate monitor monitor transmitter.
Also a look at and some attempted decoding of the signal with the Tektronix MDO3000 oscilloscope.
Polar RMCM-01 Receiver Module Datasheet: https://www.sparkfun.com/datasheets/Wireless/General/RMCM01.pdf
EEVblog Amazon Store (Dave gets a cut):
Electronics Info Wiki: