This concept has been used in new zealand for decades. The communications channel is probably moving to radio and possibly integrated in to smart meters. It used to/still does work on a "ripple" signal where the utility injects a higher frequency signal, such as 300 Hz, on to the power lines and then relays at the customer's houses pick up the signals and operate on and off.

They cycle through groups of customers so nobody's hot water is off for very long.

here is an excellent resource with pictures of the ripple plant and the evolution of the equipment http://www.oriongroup.co.nz/downloads/ripplesignalguide.pdf