The popular TFS add-on for administering permissions across TFS, SQL Server Reporting Services and Sharepoint has been updated to support TFS 2008.  The tool, call TFSAdmin, is available on CodePlex here: http://www.codeplex.com/TFSAdmin/Wiki/View.aspx?title=Home

Eventually, we plan to make this kind of functionality part of the product, but for now this is one of the best tools available for managing it.

Brian