If you're discounting the prestaForumConnectorPlugin which connects sfGuard and PHPBB3, then you may have to write your own connector. You should check if any resources exist on the websites of your favourite PHP forum.
One option is Invision Power Board (IPB) v3, as their developer resources explain how to integrate your own code (ie. symfony) into their system, including single sign-on: http://community.invisionpower.com/resources/official.html?category=41
I have integrated IPB v2 with symfony, and it wasn't ideal - there was no developer interface at that time to make IPB respect symfony. I had to hack around in the IPB source to make it work, but after discussing it with Invision was assured this would not be necessary with v3. This seems to now exist, which would be where I would start.
NB. IPB is not free, so hopefully this is not a sticking point for you.