I recently was playing around with SQL Server 2008 Reporting Services and switching between Native and SharePoint integrated mode. In the last change, I switched from SharePoint Integrated to Native but came across an error when opening the Report Manager, specifically HTTP Error 503: Service is Unavailable. As usual, I over analyzed it but did find the solution.
Even though I viewed the rsreportserver.config numerous times looking for a configuration issue, I overlooked the line under service called <IsReportManagerEnabled>. This was set to false in this installation. Switching this from false to true eliminated the HTTP Error 503: Services is Unavailable.
I too frequently forget the simple things to check first.