I have seen this occur when "Calendar Cache" is set in Outlook.

Look for Private settings for delegates/client
Tools | Options | Email Options... button
Advanced Email Options... button
Check the "Set Sensitivity" box for "Private"
A Private setting here will cause ALL forms Including Calendar meetings
   to be set for private. Private set forms are not visible to other
persons
   who have "view" permissions.

Calendar Cache for delegates/client
Tools | Options | Calendar Options
Look for "Always use local calendar". If this check mark is on then the
client will not update their free/busy information (New calendar entries)
until they synchronize to the Exchange Server. This only happens when
the client uses the "Exit and Log off" (not when they hit the "x") or
when the log onto mail in the morning. Be sure to turn off the check
mark for this option if there is a delegate involved.

Synchronization with OST's (improperly config'd)
If there is a delegate involved, disable Offline folders for both the
delegate and the person they are delegate for.
Tools | Services
Highlight the "Microsoft Exchange Server" service and click
   the PROPERTIES button
Select the ADVANCED tab
Click the "Offline Folder File Settings" button and make
   the appropriate changes.
For the registry fix click the link below for the Technet article Q276248.
Make sure the Delegate does not in turn have a Delegate.

Misguided Rules
Look for any rules that may be set to move items to a different folder,
or delete items, or...? You just need to really see what the rule is
doing and whether it may be a problem.

Auto-Archive
If a client has "Auto-Archive" set to automatically clean out their
calendar folder at certain intervals, then if the client has a
"recurring meeting" that they made to "Never End" or if it goes
longer than the archive is set for, the auto archive will remove the
original recurring meeting which removes all occurances too.
This appears to make meetings "disappear".

Check for Auto Processing of responses
Tools | Options | E-mail Options... button
"Tracking Options..." button
Turn off the check mark for both of the following:
   Process requests and responses on arrival
   Process receipts on arrival
Click OK
Click the "Advanced E-mail Options..." button
Turn off the check mark for:
   Delete meeting request from Inbox when responding

OL2000: Calendar Problems with Outlook 2000
http://support.microsoft.com/default.aspx?scid=kb;en-us;276248

XCCC: Meeting Is Removed from the Calendar If You Delete a Meeting Request
from the Inbox by Using OWA
http://support.microsoft.com/default.aspx?scid=kb;en-us;322193&Product=ol2002

Users are experiencing disappearing and/or duplicate calendaring items
across the board.  Using outlook versions 2000, 2002 and 2003.  Any
comments or ideas would greatly be appreciated.  Thanks!