John Curran commented in a message to NANOG:
There are companies which would like to be connecting new customers with IPv6 as we approach IPv4 depletion and then handle v translation for IPv4 site connectivity in their network i.e. customers connecting to "The Internet" via only IPv6 with the expectation of reaching all Internet destinations
While making the backbone networks dual-stack is going to be serious work,it's at least an understandable goal that operators can makes plans to hit. That's not the case with the requirement to provide transparent connectivity to IPv4 destinations via IPv6 transport. NAT-PT wasn't exactly an elegant solution, but it's now precisely what some providers are looking for (so connecting customers via just IPv6 is at least viable). Without it, every provider is going to come up with ad-hoc customer connection models with various IPv4 tunnelling and translation games once IPv4 address blocks become generally unavailable.
The irony is that the I* rationale for moving NAT-PT to historic was "to restore the end-to-end transparency of the Internet" and yet the only real chance we have to restore end-to-end transparency is to first have a transition to the IPv6 (using dual-stack, NAT-PT, and every other tool at our disposal) and then over time let present IPv4 destination sites add IPv6 for end-to-end transparency based on their actual need for it. Instead, central planning may have effectively killed the very tool that's needed to allow providers to provision new Internet customers over a pure IPv6-only model, and create the right motivation for existing Internet sites to go dual-stack and actually gain "end to end transparency" via IPv6.
In a nutshell, we need to be able to provide IPv6 connected users with a transparent view of all Internet resources regardless of whether those resources use IPv4 or IPv6 access. And we also need to provide IPv4 Internet users with the same kind of transparent view. Without this last capability in our networks, we will not be able to convice hosting customers to use IPv6 network access services.
Implementing the transition strategies documented inwill only solve part of this puzzle. To make a fully transparent Internet access service will also require implementing protocol translation (NAT-PT) or proxy services (ALG). Some resources for this are available on the Operational transition information page, but this is an area that needs a lot more effort to trial technologies and document how best to implement them. The existing technologies are discussed on the Relay Services page.
Some have suggested that a good way to do this work would be to prepare and plan for supporting an Internet community meeting such as IETF or NANOG with only IPv6 Internet access. If transparent access is successfully deployed, it means that all the IPv4 users at the meeting, and at home, will not notice the fact that there is no IPv4 Internet access at the meeting. This goes beyond tunneling IPv4 over IPv6 because applications will make DNS queries for the wrong kind of record (A or AAAA) in order to make a connection. These queries need to be spoofed in order for successful transparent communication to occur.