the way to address the disparate communities that form around forums and mailing lists is to make the web forum be a view into the mailing list (or the mailing list be a view into the forum, but I think that's harder)
there are some forum software packages that support e-mail (I understand FudForum is a popular one)
there is also software like nabble (www.nabble.com) which libreoffice uses to be a web version of their mailing list.
I'm investigating nabble right now for an organization that needs good communication between nntp, e-mail, and web users and it's looking pretty good in terms of functionality.
it's not currently opensource, but they seem to have plans to open it.
I'd also be very interested in learning about other alternatives to nabble.