Ran into an issue today where I needed to convert a Maildir containing over 50k messages to mbox format and couldn't really find any good tools. So I found this blog post about using mutt to do it. Unfortunately, it converted one Maildir to another. =( One crucial piece is missing, so here is the one-liner in all its glory:
$ mutt -f Archive/ -e 'set mbox_type=mbox; set confirmcreate=no; \ set delete=no; push "T.*<enter>;sarchive<enter><quit>"'The above converts the Maildir Archive/ to the mbox named archive. It also makes the assumption that mbox_type is set to Maildir (which is how I have it configured; if it wasn't for that, the default is mbox and that's what it would have created, but I changed the default to Maildir, hence the need to be specific). Line break was added for clarity.