Few things you need to consider before letting the users use Reporting Services within a DMZ environment where the Servers are exposed to the INternet and to safeguard the servers from hackers. In this case you must be aware that SQL Server uses TCP port 1433 and UDP port 1434, change it to a different port number and also considering the Browser service security too. Finally don't forget to secure the password if you are using Certificate (secured) based authentication then refer to http://blogs.msdn.com/tudortr/archive/2005/11/03/488731.aspx...(
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Read the complete post at http://sqlserver-qa.net/blogs/tools/archive/2007/10/02/configure-reporting-services-on-a-dmz-environment.aspx