If you are developing Reporting Services extension assemblies that require <appSettings> elements in application configuration file (e.g. <add key="SqlConnectionString" value="Integrated Security=SSPI;Data Source=(local);" />) then you will need to modify a couple of files:

• To design reports successfully, add the configuration to the VisualStudio app.config file
  C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE\devenv.exe.config
  
• To deploy successfully, add the configuration to the ReportServer web.config file
  C:\Program Files\Microsoft SQL Server\MSSQL\Reporting Services\ReportServer\web.config