http://meta.wikimedia.org/wiki/Talk:IPv6_initiative#Domain_servers
Because ns(0-2).wikimedia.org are missing IPv6 addresses, DNS lookups on IPv6-only clients fail to reach the Wikimedia domains, according to that thread, which makes sense because they are unable to reach a Wikimedia nameserver.
Version: wmf-deployment
Severity: normal
Answer:
"That's because we do GSLB / geo load balancing based on IPv4-only geoip data. Until that can change and work well enough with IPv6 too, we can't put IPv6 addresses on our NS records and have our DNS auth servers answer on IPv6, unfortunately."
IPv6 geolocation databases do exist, but no reliable sites exist to query such databases, so the WMF would have to host its own. Hhm...
We don't query external databases for both privacy, performance and resiliency reasons. MaxMind does provide an IPv6 database but it's of subpar quality compared to their IPv4 database (that we already use).
However, there's a larger issue: our DNS infrastructure needs to be changed; we currently use PowerDNS' GeoIP module (which was written by Mark specifically for this use and merged upstream) which doesn't support IPv6 at all. It's not just a matter of replacing a file on the filesystem.
So, it's definitely our agenda, as part of some larger pending DNS changes, but it's not something that can be easily done.