Haris Andrianakis wrote a post on his blog detailing his power line communication system:
The main purpose was to implement two devices communicating each other, using the technology of power line. Specifically, at this project the communication has been used to send data from a weight bridge/ weight machine to a remote display. For the power line communication has been chosen the ST7540 modem. ATmega168 is holding the protocol stack and controls the modem. The data are sent to ATmega168 via UART which converts and transfers them using SPI to ST7540. ST7540 is responsible for the Power Line transmission.
Check out the video after the break.