i plan to switch from uw-imap to dovecot on a couple of medium size systems and so i have written a conversion tool that understands mbox, mbx, and mix formats and converts to maildir format while trying to preserve uid info, keywords, flags, subscriptions, etc. it seems to work for me now, but i think some feedback might be useful. so if you have some files to convert and a desire to help, please download:

http://www.spiretech.com/~joey/uw2dovecot.pl

and give it a whirl. it is very sparsely documented right now but isn't too complex (yet). by default it copies your INBOX, mail dir, and .mailboxlist info into Maildir. those filenames can be overridden. i'd suggest running with -v to watch it. it requires that the output directory not exist before starting, so it should be relatively safe. it doesn't lock any files being read, so an active uw tree could cause it indigestion. it's designed to run while inbound mail and pop/imap sessions are disabled. it uses a few perl modules, but they should be standard with perl itself. it does do everthing in memory, so the largest message you have will be in memory and will be duplicated at least once as crlf pairs are replaced. also, this assumes you'll change the uid format in dovecot.conf to %08Xu%08Xv so that it doesn't change uids for the clients.

send feedback to me and cc the list if you feel it useful. once this is stable, i'll get it added to the dovecot wiki.