This is happening with my CEO and his admin assistant. She has access to his calendar of course. When she creates a meeting appointment and places it on his calendar it shows up, and then, I've watched this happen, in about a minute the item will disappear completely. They are both running Outlook 2010 on Exchange 2010.
Outlook build: 14.0.7113.5005
Exchange build: 184.108.40.206