At one time IPv6 was supposed to have built-in security. I don't know how it was supposed to be implemented, but my guess is that perhaps, if your stack and application supported it, you could simply click a button which would secure the connection in an opportunistic way, or connect securely but fall back to insecure if not supported, similar to how the Philip Zimmerman's zfone works.
What happened to this? Is it in use, or is it an optional feature, which could be implemented down the road? Is it a standard, and what is the name of it?