Thank Lowell, that's were I'm having the issue, the report isn't taking the other multiple values passed in the URL into account when rendering the report.

http://domain.com/ReportServer/Pages/ReportViewer.aspx?%2fNew+Opportunities&rs:format=PDF&rs:Command=Render&StartDate=9/7/2010&EndDate=10/8/2010&UserID=1121&UserID=1074&UserID=1106&UserID=1084

So in the above example...