http://freshmeat.net/projects/imapsync/

imapsync is a tool for facilitating incremental recursive IMAP
transfers from one mailbox to another. It is useful for mailbox
migration, and reduces the amount of data transferred by only copying
messages that are not present on both servers. Read, unread, and
deleted flags are preserved, and the process can be stopped and
resumed. The original messages can optionally be deleted after a
successful transfer.