Apple has published a Knowledge Base article that instructs users in importing Contacts from Entourage to the Mac OS X Address Book.
Apple instructs customers to create a new folder in the Finder on the desktop called Contacts.
The next step is to open the Address Book in Entourage, select the contacts to be exported, and drag them to the Contacts folder on the desktop. This process creates vCard files for each selected contact.
Users should then quit Entourage, open the Address Book application and choose the View Card and Columns options from the View Menu. After this operation, drag the Contacts folder from the Desktop to the groups column of the OS X Address Book. Once this operation is completed the contact information should be available in the Address Book.
Apple also suggests employing AppleScript to perform this operation.