One of thing I'm doing these days is to try and keep all the 35.000 email I received and sent in the last 10 years. So I looked at how to move the contents of my pst files to the standard mbox mail format.
Looking on the web I found a quite interesting explanation on the Microsoft KnowledgeBase:
ENT2001: How to Use IMAP to Move Messages Between a PC and a Macintosh in Entourage, Outlook, or Outlook Express
What the KB article says is that you just have to create an account on an IMAP mail server, configure both clients to get emails...