Nike+iPod Dissection is a great page dissecting and describing how the Nike-iPod wireless jogging datalogger works. The show insert appears to collect footfall data with a piezeoelectric speaker and transmits the data wirelessly to a transceiver plugged in to the iPod. The wireless transmitter is the Nordic Semiconductor nRF2402 ultralow power 2.4GHz transmitter. On the iPod end it transmits to a nRF24L01 transceiver (presumably only be used as a receiver).

Nordic Semiconductor make a series of very interesting low-cost, low-power ICs operating in the unlicensed 2.4 GHz bands as well as other bands. See nRD24H1 Remote Control Reference Design for one of their reference designs (available as a prototype system).

There is an interesting review article on the security implications of devices like the Nike-iPod here: Devices That Tell On You: The Nike+iPod Sport Kit