"defaulting" to IPv6 is something that is done on the DNS/client end. Verizon is so high because they have a brand new network that was built from the ground up

"defaulting" to IPv6 is something that is done on the DNS/client end. Verizon is so high because they have a brand new network that was built from the ground up with mandatory IPv6 support for all devices required. See the presentation notes that ejdyksen posted above for more information. For mobile networks IPv6 has a better business case than most networks. Mobile networks are new enough that they don't typically have large blocks of addresses ready to use so every IPv6 packet is one that doesn't take resources on your expensive CGN gear. T-mobile's wireless network is also fully IPv6 enabled.