To add a reference to SSRS 2005 in Visual Studio, you’d specify the URL to the Web Service definition file, for example
http://server/ReportServer/ReportService2005.asmx or equivalently http://server/ReportServer/ReportService2005.asmx?wsdl. You can use a shorter alternate URL, http://server/ReportServer/ReportService.asmx. If you open these URLs in a Web browser, you’d be able to view the content of the ASMX file.
With SSRS 2008, I’d expect to use the same URLs, replacing 2005 with 2008 where applicable. Alas, it doesn’t work that way. If you browse to http://server/ReportServer/ReportService2008.asmx, you’d get a directory listing rather than the expected Web service definition. A quick check of the MSRS10.MSSQLSERVER\Reporting Services\ReportServer directory reveals that the only relevant Web service definition file is ReportService2005.asmx. Further, the ReportService.asmx file has been inexplicably removed.
So in order to add a Web reference to SSRS 2008 in Visual Studio, you’d have to use the same Web service URL as for SSRS 2005, http://server/ReportServer/ReportService2005.asmx. This is somewhat confusing, but it implies that 1) there are no changes in the Web service, and 2) if you upgrade, you won’t have to update the existing Web reference.