I'm running a IPv6-enabled network, for a few years already. Internet connectivity goes through a SixXS tunnel. DNS services are provided by a bind instance. The machines on the local network have both IPv4 and IPv6 DNS entries there. Bind does also act as DNS cache, and of course it is reachable with both IPv4 and IPv6. dhcpd hands out IPv4 addresses to everybody, radvd does the same for IPv6, both point to to the bind instance for DNS service.
Recently I've purchased a cubietruck so I can play around with arm virtualization. Installed & configured the box, created fedora arm guests. Logged in, updated the virtual machines (loading updates from the fedora mirror network), all worked just fine. Then, after a few days, I've figured that dhcp was not turned on in the guests. They ran without IPv4 connectivity, and I didn't even notice!
Simliar thing happened with my jenkins build host. It somehow lost IPv4 connectivity. Not fully sure why, but there was a power failure and the dhcp server wasn't available for a while, guess this was the reason. Nevertheless almost everything continued to work just fine. Jenkins performed builds, I could see the results in the web gui. What didn't work was pulling github repos, because github can't be reached using IPv6. Needless to say this took me a few days too to figure.
Is amazing how well things are working meanwhile with IPv6 only.