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If you sync with a WebDAV server (such as Omni Sync Server), you can simply tick the box:

Reminders: [√] Publish Due reminders as a calendar

and then click the button:
( Subscribe in iCal )

Every time OmniFocus syncs, it will update that calendar to have all items due in the next two weeks. Then iCal/Calendar will also automatically sync and pick them up.

This thread links to what seems to be an OmniFocus to Reminders export script.