This  is what I got

Neither in Beta2, nor now, could you have cast a DocumentDesigner to
WebFormsRootDesigner.

My first guess would be that the user's control gets an object that
implements some interface (maybe IWebFormsDocumentService,
IResourceUrlGenerator, or IContentResolutionService), where in Whidbey it
was the WebFormsRootDesigner that implemented that interface. That would
make the cast possible but not advised. Those interfaces are now implemented
by the DocumentDesigner, so the cast wouldn't work in VS2008.

If the user is trying to get the WebFormsRootDesigner, the correct way to do
it would be to get the IDesignerHost, and then do this:

IDesignerHost designerHost =
Component.Site.GetService(typeof(IDesignerHost)) as IDesignerHost;

WebFormsRootDesigner rootDesigner =
designerHost.GetDesigner(designer.RootComponent) as WebFormsRootDesigner;