I had to add SQL Server Reporting Services to an existing SQL Server 2005 installation. This shouldn't be a big deal. Mount the SQL Server installation image, run the setup and follow the installation wizard to add extra components. Arriving at the step where I should be able to select Reporting Services, the Reporting Services option was grayed out.
So to make a long story short, make sure IIS is installed and that the World Web Web Publishing service is running!