I happened upon this article while attempting to create a method of creating data driven subscriptions in 2005 Standard. You can find the parameters list for your report in the dbo.Catalog table in the ReportServer database. The list is in XML format, so you'll have to do your own parsing. Also, I recommend using the stored procedure "dbo.GetReportParametersForExecution" as it will save you time recreating the wheel. The table dbo.Subscriptions stores the parameters in xml format and contains a name value pair that will be passed to the reporting engine.