I have found (Outlook 2003, Exchange 2003) tha if the
meeting organizer updates a meeting and sends the update
to attendees (who have already accepted the meeting), if
the attendee deletes the email regarding the update, the
meeting is removed from his calendar.  This may be part of
your users' problem.  Can anyone confirm that this is the
default behavior?  If so, anyway to turn it off?

thanks