In SSRS 2008 R2 adding a favicon to the ReportManager app was completed simply by adding your ico file to the ReportManager folder wherever that was installed on your report server.
The same technique does not work for SSRS 2012. Does anyone know how to get a favicon to display in the ReportManager app for SSRS 2012? Having a different icon for test and production SSRS installations really helps our users visually know where they are, it will be sad to see this disappear when we upgrade to 2012.