I've been working on the IPv6 core support in a cacheboy branch (Changes: http://code.google.com/p/cacheboy/source/list?path=playpen/sockaddr_change) and it seems to be coming along swimmingly.
The current goal is just to get basic IPv6 support into the base libraries and keep the rest of the codebase IPv4-only.
I've converted commBind(), comm_connect_addr() and comm_accept() to my new IPv4/IPv6 address type and nothing seems amiss at the present time. comm_open() and comm_openex() will take a little more time as there are plenty of places which create a new outgoing socket.
My next move is to modify my tcp proxy to listen on both IPv4 and IPv6 incoming ports and proxy to an IPv4 destination. I can then fire off some HTTP clients at it and see what happens.
(I may have to modify apachebench-adrian to support IPv6 though; I'm not sure what other stupidly-high-traffic open source http benchmarking clients exist at the present time.)
I hope to get all of this sorted out in the next week or so and head over to the Sydney Squid developers meet with my "alternate" IPv6 core for Squid-2 and better understand the IPv4/IPv6 requirements before discussing them with Amos.