Since I abandoned Fastmail for Gmail’s intuitive keyboard shortcuts and conversation views a couple of months ago, I’ve been putting off the task of backing up the old account. I ran across offlineimap today, which has simple configuration and seems to be doing a good job of syncing about 500mb of email.
Update 12/9/2006: I found an even better way to get IMAP backups with the aptly named imapbackup.py script.