Here’s a very brief article about scenarios for transitioning to IPv6. Its focus is pretty much limited to summarizing the likelihood that IPv4 will continue to be the de facto base standard in parallel with gradual implementation of IPv6 and then with IPv4 retaining legacy support in the future. The article gives the analogy of continued parallel support of “old school” phone systems alongside VOIP — the latter doesn’t immediately supplant the former, but is developed (somewhat) in clean-state and gradually adopted.
It’s very unlikely the Internet will ever see abrupt, sudden changes because of the amount of resources tied up in existing (legacy) hardware and software. The pace of innovation tends to exceed the pace of adoption. This leads to more evolution than revolution in the way we do things.
Edit 20070417: This outline shows challenges along with current fixes and clean-slate solutions.