I scoured the web looking for the best way to convert my wife’s email from Outlook on Windows to Mail on Mac. It turned out that the best option was to attach to a .mac IMAP account, create a folder for the outlook mail, drag all the Outlook folders into that new folder. Which copied the individual mails to the IMAP server. Then when you attach to the .mac account on the Mac and drag them into a locally created folder they migrate effortlessly. I spent a bunch of time trying the suck the mail into Thunderbird which creates *.mbox files, but when those were imported into Mac mail, all the messages were glommed into a single message in each folder you convert. That sucked!
Try this Google search if you’re facing this dilemma also. The most helpful results were at www.macosxhints.com, followed by www.uneasysilence.com for the less helpful method. The one caveat is that all the attachments get converted to that damn winmail.dat format; but that’s a problem for another day.