Have you looked at PHPNuke ?

I am not the developer of my team so I don't know firsthand. But, I have seen PHPNuke used in various places to create the whole web site.

I know that one of the PHPNUke modules is Forums and they work very well. I don't know if its possible to use just the PHPNuke Forums module though without PHPNuke making up the main web site though.

Hope that helps-