This seems to be new since the customer service people at Comcast don't seem to be aware of it when I called.
It appears that now, ALL static IPv4 assignments are automatically coming with static IPv6 assignments.
Note that in order to obtain a static IPv4 or IPv6 address you must have a business account.
To view your static IPv6 assignment go to:
Under Manage Services, click Internet
On the right hand side, click View Static IP
Login to your cable modem (use the usual http://10.1.10.1, userID cusadmin, password highspeed)
For a Netgear, on the left click LAN
Click IPv6 Setup tab on the top
make sure the IPv6 prefix and system delegated IPv6 address matches the static IP you got.