Just hours ago I upgraded my external router from PFSense 2.0-RC to 2.1 (2.1-HEAD), which enables me to have inline IPv6 support without nasty hacks. I believe Seth Mos did a great job in getting this going! Most important options are there and I had setup the options within a few minutes. I need to play a bit with dhcpd6 though, but that works fine now. This post is written over the IPv6-TCP transport, so a clear sign that it works!
I would suggest you to try it out and help us make pfSense even better!
I bet you want to have more information on this, the pfSense team has a documentation page which demonstrates how to set this up properly here
20110811: Edit: Jimp reported that it’s actually 2.1-head instead of 2.0, change that in the title and text (leave the permalink in place though), add documentation link.