I have a hard time believing that your average IP net manager
wouldn't prefer to run an IPv6-IPv6 NAT box (lets call this
NATv6) at her ISP boundary rather than do site-wide renumbering.
some of the applications that will motivate the deployment of
IPv6 will be those that do not work with NAT. (if they do work
with NAT, it's probably easier to just use IPv4.)
so it's quite likely that whereas imposing a NAT box on IPv4 users
wouldn't break many apps using IPv4, imposing a NAT box on IPv6 users
will break many of the apps using IPv6.
also, people using NATs are starting to become aware of the problems
that they cause - particularly those trying to build VPNs between
multiple NATted networks. so by the time there's enough v6
deployment for people to have to renumber, enough people will have
been burned by NATs that won't want to repeat that experience.
Keith