Now that IPv6 is up & running, several things are not though:

OpenSSL lacks IPv6 support, despite a patch being available for some time now. But GnuTLS can do it:

   $ echo | gnutls-cli -p 443 ipv6.google.com
   Resolving 'ipv6.google.com'...
   Connecting to '2001:4860:4001:803::1011:443'...
   - Certificate type: X.509
   [...]

Most of the Debian mirrors are not reachable via IPv6. Wow. Not even kernel.org (except for a few hours on IPv6 Day). I remember many of the NetBSD mirrors are IPv6-ready.
With that in place, apt-get is of course unable to complete its task. And even with an IPv6 enabled mirror in place, security.(ipv6.)debian.org seems to be unreachable from time to time, like today. And apparently there's no way to tell apt-get to stick to IPv4 for now. One can set a sitewide preference though and even prefer IPv6 for specific hosts only via /etc/gai.conf.