IPv6 on the Internet and IPv4 inside
That's the future and those who gave IPv6 to the world should start working on making it reality.
Let's all face it, no sane management will spend (huge amount of) money and accept to put their legacy mission critical systems at risk just because Internet runs out of IPv4 addresses.
The happy bunch of IPv6 lovers must realize that in large enterprises there's simply no business case for such a migration. Yes, IPv6 is modern, more efficient but the potential impact on back-end systems can be huge and business continuity always comes first.