Where did they go? Hmm, they are still in the same namespace, but they have been moved out of the Microsoft.SharePoint.portal.dll. They are now located in the Microsoft.Office.Server.FilterControls that is only located in the GAC.
Check out my reply in the MSDN Forums.
Chris
META: <type
name="Microsoft.SharePoint.Portal.WebControls.QueryStringFilterWebPart,
Microsoft.Office.Server.FilterControls,...>