MobileMe: Moving events between calendars on iOS may cause the event to not sync with me.com/calendar, iCal, or Outlook.

When moving an event from one calendar to another as follows: Launching the Calendar app on an iOS device that has a MobileMe account set up with at least two calendars (such as Home and Work). Selecting an event in a Home calendar. Moving the event from Home calendar to Work calendar. You may see: Calendar events duplicated on me.com/calendar or in other applications syncing your calendar data (such as iCal or Outlook). Calendars out of sync on me.com/calendar or in other applications syncing your calendar data (such as iCal or Outlook).

Source: Apple - Support - Most Recent - iPhone