BuddyPress.org

There’s lots of ways to build bridges between platforms, but no one usually does it until they need to do it themselves.

I’ve never seen ccHost until now, and as powerful as it looks, it really looks like a competing project compared to what WordPress/bbPress/BuddyPress already attempts to do, with the help of plugins of course.

People have built integrated logins for phpBB and vBulletin, so I imagine someone could do this for you too.