What made you think of doing this, what are the advantages, is this common in other setups?
Linode does not charge for IPv6 traffic inside of their network. So it made sense to put all of the back end traffic on IPv6 and save our network quota for use on the front end.
Its not a common setup to say the least, and sanity was questioned on it. One of our sysops guys was on VAC while we did this, and when he came back the response was basically "WTF?". The problem is our offsite backup is in a data center in France, and with the old IPv4 setup, we were looking at the possiblity of having to run a VPN and NAT. We could get around it by creative firewalling, and stupid DNS tricks, but I was sick of dealing with those from a previous job. Furthermore, I'd like for us to have mirrors in multiple data centers across the world, and IPv6 addressing means that no matter where a node is, it can always access another node with a consistently known IP address, and rdns/dns *just work*. No stupid hacks, no insane IPtables routes. It Just Works.
It might be kinda extreme, but it puts us very much ahead of the curve on such things, and our network is extremely nice to work with due to the way its setup as an end result (I've had a couple minds blown on how we do single signon/LDAP SSH/etc.).