An Informational draft RFC by John Curran was recently published, outlining an IPv4 to IPv6 transition plan. The paper provides a clear guidance to organizations regarding specific
expectations that change over time, and vary greatly by organization. A
timeline of the different phases was set with the intention of allowing
enough time for the necessary planning and deployment steps which each
organization must undertake. The author proposes the transition to predominantly IPv6-connectivity by Januaray 2011 in response to meeting the overall requirements of allowing the Internet to scale as
specified in "The Recommendation for the IP Next Generation Protocol"
On the contrary, Randy Bush provides a very informative presentation, IPv6 Transition & Operational Reality, regarding the reality of such a transition. The presentation discusses the different myths about IPv4 and IPv6, the emergence of a market for IPv4 addresses, and the transition from allocation to entitlement among others.
For more background data and interesting comments from Geoff Huston, read his IPv4 Address Report or his ISP column articles on The End of (IPv4) World, and Transition to IPv6.