After installing Tiger and importing my mail from my Panther backup, I was upset to see that the "Sent Items" mailbox did not import. I keep all my emails, and the Sent Items mailbox was of value to me. Luckily the solution to this problem is an easy one:
In your Panther (or earlier) backup, go into the folder ~/Library/Mail. There you will see a bunch of files, and two important folders: "Mailboxes" are all your custom mailboxes, except for inbox / outbox / sent-items, which are stored in the "POP-your_name@popaccount.com" folder (assuming you used POP for mail -- I imagine IMAP would have a similar naming structure). If you copy the "Sent Messages" folder into the "Mailboxes" folder, and then re-run the Import in Tiger mail, you will now see "Sent Items" in the list.
Once this has been imported, it's simply a matter of dragging all the messages from "Import/Sent Items" into your real Sent Items folder and voila!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050502032936184