I have had all calendar entries disappear twice in the last week, no known reason. Look at the calendar and there are no entries. Since I use Exchange, my calendar is cleared on the Exchange Server as well and then propagates to my iPhone.
At first, I thought this was related to my sync of iCal with Entourage, but removed that sync after the first disappearance. The first time, items were not visible in the deleted items, but the System Cleanup from IT said there were 433 items removed from my deleted items folder. I was unable to recover these items. I rebuilt my calendar from a Time Machine backup.
After three days it happened again, right before my eyes, I saw items disappear. I removed the network connection before it was able to sync with my Exchange Server this time. iCal was fine as well. All calendar items disappeared from Entourage.
I was able to resync my old Outlook with Exchange and then sync Entourage. Most everything came back, but two items that were accepted during the disappearance.
The last update that was performed was the Office update to 12.1.2. No other software has been added or changed.
This happened a few weeks back to my contacts list as well, but thought this was an issue related to iPhone syncing. Maybe not.
Stuart