Migrating Linux/cyrrus IMAP into Lion server dovecot
There's a Cyrus2Dovecot migration tool developed by Freie Universität Berlin.


  Cyrus2Dovecot is a full-featured command line tool for converting the e-mails of one or more users from Cyrus format to Dovecot Maildir++ folders. It allows for performing a server transition which is fully transparent to both POP and IMAP users.


And other migration solutions in the Dovecot wiki.