I've thought about doing this myself using the Hunchentoot and Elephant libraries. I've never used UnCommon Web but I see it's worth looking in to. I remember looking at the phpBB table layout, writing BBS software in Python at the time. Anyway, from that experience, writing basic forum software isn't very hard. I like phpBB, and it's code base is very mature. We could strive to duplicate most of it's features and see what happens.
I thought of that as well; the only thing that stopped me was my relative inexperience with web programming, in Lisp or otherwise. Now that I come to think of it, it would make a wonderful learning opportunity.