[Greg Jones]

iCal can work with 2 types of data-calendar events and actions (to-do items, or tasks) while OmniFocus works exclusively with actions. Actions in OmniFocus will sync both ways with to-do items in iCal. However, once you change the to-do in iCal to an calendar event by dragging over to a date on the calendar, it will no longer sync to OmniFocus as it is no longer an action.