Henrik Gemal has written up a clever tutorial on how to backup IMAP mail using Mozilla Thunderbird. Follow that up with the corresponding tutorial on restoring IMAP emails.
Now those of you who're still using POP3 to get your email no longer can say "IMAP sucks because it doesn't allow me save my emails and I have to delete them otherwise my inbox will be full".
In related news, Omar Shahine, who co-designed Microsoft Entourage, says in his blog entry on IMAP, Thunderbird, and mail clients that:
Thunderbird is an almost perfect IMAP client for Windows. If you use IMAP, this is the product for you.