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. 

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