Nice. It just occured to me that Netscape never released v5 of their browser either. This was after they open sourced it. They just jumped straight to 6 (officially). Well... atleast the mystry of ONE disappearing v5 is solved ;-)
We had a presentation given to us from a lady from IBM responsible for IPv6 in IBM's AIX OS. Here are her slides. It's good stuff... provides a nice overview. Fun fact she gave us - IP v6 has enough address capacity to represent a computer in every squarefoot of earth.