I noticed yesterday that Comcast loaded a new firmware into my Comcast provided modem, and it does show IPv6 settings now, but they are not yet enabled. This is in Jacksonville FL with a business account and an SMC modem/router combination.
At the same time (probably) and the reason I checked the modem, my connectivity became pretty poor. The modem keeps getting very slow about every 1-2 minutes with pings to the modem itself exceeding 1sec. Also, the web based admin interface doesn't respond well at all. The modem is also not listed as IPv6 capable at »mydeviceinfo.comcast.net
(but maybe they only consider the old firmware). I suspect they may be trying to do too much with this modem.