The Exchange Server 2007 SDK provides the both Microsoft.Exchange.Data.iCalendar and Microsoft.Exchange.Data.Tnef
These should give access to read and write calendar items.
You will need to grant the account you are connecting with the proper rights. Information on setting service account rights via PowerShell can be read here: http://msexchangeteam.com/archive/2006/11/28/431665.aspx
- Joel