Shamelessly ripped from O’Reilly…

Microsoft has not yet provided an import function that allows bringing email messages into Entourage (.mbox files) from Outlook (.pst files)
Here is a procedure, which works moving message files from Outlook (or Outlook Express) on Windows XP to Entourage on Mac OS 10.2.
On the PC
1) Open Outlook Express and import the mail folders from Outlook: File/Import/Messages. (This converts the messages in the Outlook .pst file into .mbox format. Using the Outlook Express import utility has the advantage of maintaining the date of the email, which would be lost using the export to a file function within Outlook.
2) In Outlook Express, open an email folder, select messages and drag and drop into a Windows Explorer folder. (The messages will be appended with an .eml extension but they are in a Mbox format readable on Mac).
3) Transfer the folder to the Mac (or open the folder on the Mac if the PC and Mac are networked).
On the Mac
4) Open Entourage. In the Mail folder create a temporary folder and drag and drop the Outlook Express .eml files into the folder (do not try to transfer more than 150 files at a time).
5) The messages are now in Entourage, but each is in a separate folder. To put them in the appropriate folder (such as Inbox), you can search for and select all the messages, then drag and drop into the appropriate folder. Here’s how a) click on the temporary folder; b) go to Edit/Advanced Find from the drop down menu; c) Check the “Include subfolders” box and under Criteria select “Does not contain”, then put in any text not in your email subject line (for example: qqqqqqqqqqqqqqqqqq). All messages should be selected and you can drag and drop to the appropriate folder. Delete the temporary folder, which will now be filled with empty folders.