Comcast/Netgear has finally released the IPv6 firmware for the Comcast supplied WNR1000v2-VC router. The new version is 18.104.22.168NA, and it can be downloaded from »support.netgear.com/app/answers/···id/20757
(the automatic upgrade search in the router does not yet find this firmware version, but the instructions at the link explain how to install it manually).
FWIW, I could not test it for functionality with Comcast's IPv4/IPv6 dual stack because my area does not yet have it.
EDIT: Removed a false alarm report of outbound PPTP pass-through with this firmware version. A pushbutton factory default reset got it working (perhaps a word to the wise should anyone else see problems after installing this firmware upgrade). --
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.