I've seen this suggestion some time ago on this mailing list.
I've started this project, you can have a preview here :
It's a small mailing list in french, there are still some
bugs, but you'll get the idea of how it works.
It's not beta, it's still unreleased development. My main
problems are all the date stuff, and the algorithm to
build threads when the reference headers are missing.
It's still on development, I decided to start working on
this alone instead of looking for coworkers not to spent
years discussing the architecture, and to decide.
I read from Earl that mhonarc can't evolve that way, and
I really don't like perl so even if it's a waste I can't
reuse stuff from mhonarc, I preferred that.
It's in python, either the cgi scripts which dynamically
generate html-only pages and the command-line utilities.
The database is currently MySQL, I plan to have a PostgreSQL
interface working for the initial release. I doubt it can
work correctly without a full-featured database engine.
Look at the homepage of onesite.org again in about 3 months
and I'll have something ! Probably GPL, and ready for
cooperative developments with a cvs and so on, maybe on
sourceforge if it works better.
I know that everyone doesn't like the idea of dynamically
generated contents, so there will always be a place for
both type of projects.
So I'm not asking for help, just suggesting to hold your
breath if you were about to start something similar.