[mtbrain]

In my experience it has been a corrupt contact in Exchange. Check your Sync Issues folder in your Outlook client for any errors. If you are able to determine what is failing you can go in and fix that issue. However, if you are not that lucky my next suggestion would be to start by syncing one data source at a time (email, tasks, calendar, contacts). When one source syncs successfully then add the next one.

In ActiveSync on the PDA does it give an error code?