I recently decided to move our reporting services running on our SCCM site server to our more beefier SQL Server, I was also wanting to do this for our Forefront Migration (as Reporting services is a requirement). So I created a new server under Site Settings -> Site Systems and put our SQL server into there. I then added the ConfigMgr Reporting Point and Reporting services point with SCCM adding in Component Server and Site System into the roles.
After a little while I then checked the Site Component Manager logs and found the following error:
SMS Site Component Manager could not access site system "<servername>". The operating system reported error 2147942405: Access is denied.
So I went looking to make sure that the server was up, and our Administrator account had local machine rights, it was there.
After some further investigation I also found out that the SCCM site server also needs administrative rights, so I added the computer to the local administrator group and restarted the site component manager to force it to retry installing the components and the error disappeared and SCCM began installing the components onto our SQL Server.