BTW, I just converted my first mbox to HTML using MHonArc. There are some things I don't like (where are the by-author and by-date archives? Can I split my months and give an overview similar to Pipermail? Perhaps I can and haven't read enough of the manual...)
MHonArc won't split by month (it expects you to do this beforehand), but you can create author/by-date archives using rc files. I usually pre-process with procmail to do the splitting (and create an archive of the actual Berkeley-style mbox in case I need to recreate the archives, etc.)
Take a look at
for lists in action, and
http://www.mallorn.com/~lindsey/mhonarc/
for the rc files that make the definitions that you're looking for. They're old though -- our newer ones are site-specific based on local patches.
Chris