World IPv6 Day is over, but we'll continue to serve AAAA (IPv6) records for 2.pool.ntp.org (and 2.europe, 2.fedora, 2.debian, etc).
So far no problems have been reported, even the servers going through IPv6 tunnels seem to work fine.
We already have almost 200 IPv6 servers in the pool; though that's less than 10% of all the pool servers and they're mostly in a handful of countries versus the wide deployment we have for IPv4.
The pool system is tracking, but not yet showing separate stats for IPv6 servers by default. (You can however "hack" the graph URLs to show them by adding -v6 to the file name, for example the European counts.
You can see the growth in IPv6 servers available in the pool on the zone pages.