My answer was based on your original post because what I now understand you are trying to do is a hack using SQL Server 2005 relational engine to run SSRS 2008. That is possible however I have not seen separate server or cross domain implementations.
It is generally used by developers running Vista editions which comes with IIS7 without Windows authentication which is required for SSRS 2005. So check the link below go to the last comment and you will see
install SQL Server Reporting Services 2008 only (not database engine as well) and created the ReportServer database on my SQL Server 2005 instance using the SSRS 2008 Configuration Manager.
Kind regards,
Gift Peddie