Permissions
When you make a change to your calendar, an event fires that reads through all the items in your calendar and composes a free/busy message which is sent to a public folder. When you view free/busy times, you are actually viewing a message in the public scheduleplus free/busy folder. When you hover over the times on a mmeting request with your mouse, you are actually attempting to connect to the other user's calender and read the detail. If the user hasn't explicitly given you permission to read the detail, you can't.
