It's not clear whether Comcast will be using these for internal or external use anyway; Comcast's very interesting IPv6 presentation from 2006 says that they ran out of the 10.x.x.x network for internal management and had to start assigning "public" addresses.
(It's not clear why every STB needs an address that is unique within Comcast's network. It would be possible to have them grouped into multiple 10.x.x.x subnets and use an application-level mechanism to communicate across different subnets, if you needed to at all. Certainly they are not communicating point-to-point for management purposes! I hope.)
The presentation contains a point DRC and I realized a long time ago, that L3 is not the problem--- every other layer is! So I wonder how well Comcast did on their deployment plan; the large allocation suggests they did not entirely succeed.
As a specific example of this sort of IPv6 deployment problem, Google provides service over IPv6, but not unless you specifically request it by going to ipv6.google.com. The reason is that some browsers/OSes preferred the IPv6 addresses even if they were not actually reachable. Now a horrific whitelisting scheme is being proposed to give AAAA addresses only to resolvers that are 'approved' to talk IPv6....