A friend of mine had some email in her work account she wanted to back up because she was going to give her 2 weeks notice today. (it went well!) Not only was she trying to back up emails in an Exchange server through Outlook 2003, her new computer is a MacBook with Leopard. It's nice. But you can't just copy the emails over...
I found a couple articles, one suggesting I use Microsoft's Entourage (F no!) or install Thunderbird on her XP box, synch Outlook, then transfer over to Thunderbird in Leopard. Thunderbird choked trying to import Outlook's settings, so plan C: import to Gmail using IMAP. DUH!
With whatever client is supported that you want to use, just set up Gmail to accept IMAP in that client, then drag over the emails you want to move. So freaking simple! Gmail is smart about how you move things over too. Check out the IMAP actions in Gmail page to see why it's a great solution. Flagged messages get starred. Messages in folders get applied a label. Messages in subfolders get applied Folder1/Folder2/Folder3 labels.
I'd love to switch over to Google Apps at work, and will be using this technique to transfer email.