Nokia N70 phones have a “Mark as not done” option for To Do items. If the “Mark as not done” phone option is used on a synced iCal To Do item whose status is “completed,” the status will not change in iCal.
Apple’s solution was … less than helpful:
Change the “completed” status of To Do items in iCal instead of on the phone.
Information about products not manufactured by Apple is provided for information purposes only and does not constitute Apple’s recommendation or endorsement. Please contact the vendor for additional information.