I'm experiencing the same problem. I discovered a workaround in another forum "somewhere" ;-} the following:
Blackberry Desktop Manager -> Synchronize -> Configuration tab -> Config synch... -> Tasks -> Configure -> Advanced Settings... -> Field Mapping: Now set the handheld "Start date" to NOT synchronize.
Why does this solve the problem?
On the blackberry, tasks don't have a start date. My guess is that Intellisync is finding a number it thinks is a start date, and that number is being converted into a strange start date in Outlook. In Outlook, whenever a start date is AFTER the due date, the due date is set to be the same as the start date.
This is frustrating, because all RIM needs to do to fix this problem is change the default field mapping.