Using reporting services 2005 and today, there's no Excel option in the drop down list when you choose to export the report.
I've logged onto the offending webfarm and Event Viewer has told me that the error is "Report Server Windows Service (RS2005) cannot load the EXCEL extension".
The Reportserver.config file has, between <render> tags, the reference to the extension file, "<Extension Name="EXCEL" Type="Microsoft.ReportingServices.Rendering.ExcelRenderer.ExcelRenderer,Microsoft.ReportingServices.ExcelRendering"/>" and the rendering extension file itself exists.
I'm going to restart the webfarm tonight and I hope that will solve it but this is the second time this has happened and I want to know why.