This agrees with what I've read about Comcast's IPv6 deployment to cable modem customers — they're rolling it out quickly in markets served by Arris CMTSes and not at all in markets served by Cisco CMTSes, citing problems with the Cisco gear.
If IPv6 really is undeployable on the uBR line with the current IOS releases, that's one hell of a black eye for Cisco.
Verizon shows some IPv6 support over FIOS. In the quick search I performed I didn't see any info about DSL. I've been hounding Suddenlink about their IPv6 support for some time. The response I received from one of the techs was that they were having issues with Cisco on the headend equipment.