I mentioned yesterday that I was having a go progressing through the Hurricane Electric IPv6 Certification. The main reason for this is that they do an IPv6 email test, and being that I want to be part of the Internet, and not just the legacy (IPv4) Internet, I'd decided that I wanted to be able to receive emails over IPv6, which at the moment, is not something that gmail/google apps offers.
I don't want to run my own mailserver (not entirely anyway), as I can't be bothered to worry about backups, availability, storage, spam filtering (which google are very good at), accidentally setting up an open relay, the hassles of sending email from a domestic IP, etc. etc., so I'm sticking with google as my email service provider. This left me with a little problem, of how to receive email via IPv6 until google offer this.
What I've done is set up a virtual machine on my home server, running a copy of postfix listening on IPv6 (and probably IPv4, but port 25 is closed on my router, so I don't care). This postfix is configured essentially as a backup MX, set to accept emails to my domains, and relay them all via the first of gmail's MX records.
I then just had to tweak my DNS, adding an MX record as the highest priority MX, but only setting up an AAAA record for this MX, not an A record. This way anyone trying to send me email via IPv6 will hit my 6-to-4 email gateway, and anyone who only sends over IPv4 will send directly into google's system, assuming that their SMTP server behaves properly, and delivers to MX servers lower down the priority list.
And if their mail server doesn't send to lower priority MX records, their email probably isn't worth reading anyway.