IPv6 support is available in current stable releases. See the release notes for more details. Versions of pfSense and FreeBSD
For the latest info, see the IPv6 board on the forum. IPv6 is used in production by a number of locations, including all of our datacenters. This very site as well as all our other sites are reachable via IPv6 going through pfSense, and have been since mid 2011.
By default all IPv6 is dropped since overly permissive rulesets can otherwise allow IPv6 traffic and that is likely undesirable in most cases. This behavior can be disabled under System > Advanced on the Networking tab.