One of the major reasons I want to upgrade our Reporting Services to 2008 R2 is because R2 comes with PowerPivot and SSRS report can be easily imported into PowerPivot. One way to bring report data into PowerPivot is using the Export to Data Feed button on a report.
Unfortunately the upgrade process from plain vanilla SQL 2008 to R2 have a bug that somehow excluded that rendering format and that option is missing from the reports.
After a little searching I found the solution. The following line needs to be added in the rsreportserver.config file in the Render section:
<extension name="ATOM" type="Microsoft.ReportingServices.Rendering.DataRenderer.AtomDataReport,Microsoft.ReportingServices.DataRendering" visible="false"></extension>