Comcast has reported that they have fully deployed IPv6 dual-stack across their entire backbone broadband network and are working toward reaching 50% penetration to customers by the end of 2014.  They have also stated that they are now carrying 1 Terabits per second of external to the Internet traffic calling out youtube as one of the primary sites which are using IPv6 natively.

Comcast Reaches Key Milestone in Launch of IPv6