iCal 5.x: About Exchange calendars
If your iCal calendars are administered through a Microsoft Exchange Server 2007 with Service Pack 1 or later (for example, through your workplace), you have a main Exchange calendar named Calendar, and a main reminder list named Tasks.
You can add calendars to your Exchange calendar account. However, you won’t be able to see updated invitations or attendee responses for the events in these additional calendars. Also, events in the additional calendars won’t be visible to delegates and won’t appear when other users check your availability. You can only use these features in your Exchange calendar named Calendar.