Randolph My original reason for moving my email to Thunderbird was so that I could run SUSE 10.1 on my laptop.
Since my 60GB drive packed up though, I went back to Windows. Now that Outlook 2007 is running, and there are minor compatibility issues with Thunderbird on Vista, plus most of my email prior to 2005 is still in Outlook, I've decided to go back to Outlook.
IMAPSize has a mbox2eml tool that converts all emails to eml files, which are then imported into Outlook Express or Windows Mail. Outlook 2007 then imports those directly. Voila! Out of the tens of thousands of emails I have (no, really), only about 20 have conversion problems, but are still readable.