There is IPv6 support available in the 2.1 stable release, see the release notes for more details. [2.1 release notes]
For the latest info, see the IPv6 board on the forum. This is being 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 2.1, and have been since mid 2011.
By default, in pfSense 1.2.3 and newer, all IPv6 is dropped by default 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.