See the following connect article (and vote for it to be fixed!
Bug 557655 SSRS2008 - Extra blank line in csv files created by csv renderer on export of report.
You need to alter the rendering extenstion setting for csv export in the RSReportServer.config file. Set the Device Info setting for CSV for ExcelMode to false like this:
<Render>
<Extension Name="XML" Type="Microsoft.ReportingServices.Rendering.DataRenderer.XmlDataReport,Microsoft.ReportingServices.DataRendering"/>
<Extension Name="NULL" Type="Microsoft.ReportingServices.Rendering.NullRenderer.NullReport,Microsoft.ReportingServices.NullRendering" Visible="false"/>
<Extension Name="CSV" Type="Microsoft.ReportingServices.Rendering.DataRenderer.CsvReport,Microsoft.ReportingServices.DataRendering">
<Configuration>
<DeviceInfo>
<ExcelMode>False</ExcelMode>
</DeviceInfo>
</Configuration>
</Extension>
The default for the Excel mode setting is true. There is a known bug in the Excel Mode CSV renderer that adds an extra blank line. This workaround will allow you to get around the error. I had to implement this when I moved from SSRS 2005 to SSRS 2008.