Class Item has the Id property which refers to an ItemId.  This id changes when an item is moved to another folder.  I found out that the id associated with a mailbox item that stays constant is called a GUID.  Using OutlookSpy I am able to see the GUID Exchanger server assigns to a mailbox item, say a meeting.  

It looks like you have to create your own GUID property via ExtendedProperties-why isn't this a regular property like Id in the Item class?

You can use PidTagSeachKey (PR_Seach_Key) property http://msdn.microsoft.com/en-us/library/cc815908.aspx which doesn't change when you move or copy an Items between folders. Creating your own property can also be a good idea just depends on the application. The Meeting Global Object Id will only exist on Meetings and meeting invitations and is what you should use to relate the two together.

Cheers
Glen