There are some roadblocks if you choose to install Reporting after your Management Group has already been running the SP1 upgrade. Without going into great detail, it seems the conflict is directly related to Management Pack versions already imported into the SP1 deployment and versions in the RTM installation of Reporting. I've tested this with only a minimal set of MP's currently running in my SP1 environment. If you have a number of MP's imported into your MG, there may be other MP dependencies. Either way, the installation log file should output that information for you.
So, my process was to simply install the Data Warehouse RTM and Reporting RTM. Then, to upgrade both these components to SP1. The Data Warehouse components installed fine. The Reporting part did not. I opened the installation log (choose to view the installation log file on the last dialog box of a failed installation, or by navigating to "C:\Documents and Settings\%username%\Local Settings\Temp\MOMReporting*.log").
Following are some failures I noted from the installation log while attempting the Reporting installation.
Failed to load MP C:\Program Files\System Center Operations Manager 2007\Reporting\Microsoft.SystemCenter.DataWarehouse.Internal.mp
Failed to load MP C:\Program Files\System Center Operations Manager 2007\Reporting\ODR.mp
I decided to delete these MP's, first by exporting and deleting the Default MP (as there were dependencies). I then re-imported the Default MP and proceeded with the Reporting installation. This was a success. As well, the installation imported the MP's I had deleted in previous steps.
I then proceeded with the SP1 upgrade of the Data Warehouse and Reporting components without issue. After a period of time (20-30 minutes), I saw reports populated in the console Reporting pane.
By the way, I have seen others with issues installing Reporting in a MG that is already at SP1 level and there solutions were different. This workaround may or may not work, depending on your design and configuration. This is just one example and how I went about solving this problem in my environment.