Today @hakmem tweeted a blog post by John Levine entitled «Why DNS blacklists don’t work for IPv6 networks«.
I find the last paragraph very interesting:
For the most part mail systems simply won’t use IPv6 addresses, since all the mail that anyone wants will continue to be sent using IPv4.
How do you define «All the mail that anyone wants» ?
What I want in my email, is not always what another person wants.
Why will it continue to be sent using IPv4, when every dual-stack server that runs an MTA these days *prefers* IPv6 transport when it is available?
Don't fear the penguin 
I think this presupposes that the email servers of certain ISPs and ESPs will not be dual stacked for quite some time.