If you have both Outlook 2007 and Exchange 2007, they provide a free/busy setting that shows only subject and location. That's not full calendar sharing but might be closer to what you're looking for.

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Sue Mosher, Outlook MVP
Author of Microsoft Outlook 2007 Programming:
Jumpstart for Power Users and Administrators
http://www.outlookcode.com/article.aspx?id=54
> Is it possible to let people who you have given permission to view your
> calendar see the subject and location of an appointment but not the notes
> contained in the appointment? As far as I can see if you make an appointment
> private others cannot see any of the detail.