I would guess that on fully deployed IPv6 each broadband customer gets a certain ipv6 block linked to the MAC of their modem. meaning the cable co will know any device using an IP in that block has come over that customer's connection.
A word of caution, however: as we move run out of IPv4 addresses and into the IPv6 transition, the network will increasingly be kludged with devices such as NAT boxes which may make it increasingly difficult to match a specific IP address to a specific individual.
