How much ipv6 support is included in the new RC?
By default, there is no IPv6 support in any of pfSense 2.0, RC3 or otherwise. You need to gitsync databeestje's fork in order to enable any IPv6 functionality.
This thread is a great place to start.http://forum.pfsense.org/index.php/topic,32549.msg168066.html#msg168066
It gives you an overview of setting up your tunnel and how to gitsync his repo.