Sprint has been IPV6 enable for several years. Some devices are/were duel stack, some devices IPv6 only with the 464XLAT.

You can see the adoption of ipv6 enabled devices growing: http://www.worldipv6launch.org/apps/ipv6week/measurement/ima...

All major providers ipv6 percentage can be see at http://www.worldipv6launch.org/measurements/