I actually created a fairly simple forum system as a small project while I was teaching myself object-oriented programming with PHP. I could let you take a look at it to see if it was what you would be interested in; it's sort of actually a template system with a forum module, but it could be stripped down to be used within your site.

I don't know whether it would require a lot of modification or not, though. I don't have much time now, but I'll post about it a little later if you haven't found anything.