Only universal security groups can be used for client permissions. The KB article at http://support.microsoft.com/default.aspx?scid=kb;en-us;274046 explains that you need a native-mode domain in order to have USGs

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> Is it possible to share an Outlook 2003 calendar with a GAL distribution
> list? I am running Exchange 2003 in mixed mode on Windows 2000 AD. And, I
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> changes in Exchange 2003. As I understand it in Exchange 5.5 you could assign
> permissions to a calendar via a distribution list.