See http://www.slipstick.com/exs/permissions.htm#tools for scripts and tools that youi can use to do this. There's no built-in feature in Outlook or Exchange to do it for you.

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Sue Mosher, Outlook MVP
Author of
Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> Hi all,
>
[quoted text clipped - 3 lines]
> We would like to be able to setup all exchange / outlook accounts with
> universal "Read" access to the users calendar as default. Is this possible?