This Windows Mobile Calendar problem happens all the time to me ever since Microsoft removed the two-way conflict resolution option. Here’s the scenario.
1. Create an appointment on a WiMo smartphone
2. Sync it to Outlook 2003 (or 2007)
3. After syncing, edit the appointment and change it to a different day (i.e., reschedule it)
4. Resync it with Outlook
In my case, the rescheduled appointment is removed and replaced with the original appointment from Outlook. Really annoying.