Google will be adding mail migration to Google Apps this week, along with some new features for Google Apps.
This is being promoted as especially helpful for organizations looking to better organize their emails, and migrating them to Google Apps is a great way to start. Mail will be migrated to a user's GMail account, organized into conversation threads (Google style) and made searchable (also Google style). Mail migration currently supports only those email servers with an IMAP interface. The emphasize on documents being made fully searchable is important to note in this case of mail migration to Google, as you'll probably want to first be well versed in the act of searching for emails, since you will no longer have folders to organize your documents.