The latest hotfix package for Outlook was released this week which resolves many of the .NET item leak side effects in the calendar by changing the behavior of Outlook to not reuse calendar item references in memory and instead reload them from the store. The KB article calls out a specific scenario of unintentionally saving and sending a meeting request, however, the fix actually goes beyond this scenario and I would recommend it for any .NET item leak problems involving calendar items.
957692 Description of the Outlook 2007 hotfix package (Outlook.msp): October 28, 2008
"Consider the following scenario:
•In Outlook 2007, a user has a managed add-in installed. This managed add-in is listening to an Application level event.
•As a meeting organizer, the user sends a meeting request to an attendee.
•Then, the organizer opens the meeting appointment to make some changes.
•After that, the organizer clicks Send Update to try to send the meeting update.
•During the sending of the meeting update, the organizer cancels the send and clicks Don't save changes.
In this scenario, a meeting update is sent to the attendee unexpectedly. Additionally, the changes to the meeting appointment are also saved in the organizer's calendar. "
If you have a .NET Item leak issue involving meetings or appointments, this fix may resolve it. If you are experiencing such an issue with a managed add-in then you should click the "View and request hotfix downloads" link at the top of the KB article. Please note, this does not replace the need to properly release references in add-in code.