If you're looking for a more-or-less permanent archive, you could use Exmerge to export the mailboxes to PST's. Since Exmerge can export multiple mailboxes at a time, it should be fairly efficient. Then you can move them to CD's for offline storage or designate some other disk location as your archive.
Another possibility would be to use a 3rd party backup software (Backup Exec, Arcserve, etc.) to create brick level backups of the mailboxes. However, I wouldn't recommend doing this to tape because long-term storage of tapes is not that reliable. You could designate a hard drive location for the archives initially, and then if desirable you could also move these files to CD for off-line storage.
Hope this helps!