While it doesn’t have much to do with VoIP security, I found this Internet Draft on IPv6 Transition Security Considerations to be a very worthwhile overview of IPv6 security issues, especially as those issues relate to the transition from IPv4 to IPv6. From the abstract:
The transition from a pure IPv4 network to a network where IPv4 and IPv6 co-exist brings a number of extra security considerations that need to be taken into account when deploying IPv6 and operating the dual-protocol network and the associated transition mechanisms. This document attempts to give an overview of the various issues grouped into three categories:
o issues due to the IPv6 protocol itself,
o issues due to transition mechanisms, and
o issues due to IPv6 deployment.