I plugged my cable modem into a dumb switch. It had a link, but there was nothing else attached; there was no router to act as a DHCP client, no frames being sent to the cable modem at all. In six days, my usage meter incremented by 0.5 GB. Plugging in a protocol analyzer revealed a constant stream of ARP requests for other subscribers' IPs. These were apparently counted against my cap, and presumably they were counted against everyone's caps, making 250 GB/month 1% smaller than advertised.
Comcast sent me to a document describing their metering methodology:
http://www.netforecast.com/documents/NFR5101_Comcast_Usage_M...
Their reps defended the practice of counting Comcast-generated broadcast traffic against everyone, even though there's no way for subscribers to prevent it.