Adam,

You ARE the man!  I've been trying to find out how to do this for days on
end, and am really pleased it's now working.  In order to save other people
time looking for the other thread that you posted to, I'll include the
information here:

============================================================
Add a reference to the following two assemblies:
 + Microsoft.VisualStudio.Designer.Interfaces
 + Microsoft.VisualStudio.OLE.Interop
These are available by downloading "VSIP 7.1 Extras" from Microsoft

I also added a reference to:
 + System.Design
----------------------------------------------------------------------------
--------------
Add the following code to the add-in:
----------------------------------------------------------------------------
--------------
DTE dte = (DTE)GetService(typeof(DTE));
HTMLWindow hw = dte.ActiveWindow.Object as HTMLWindow;
object o = hw.CurrentTabObject;
IOleServiceProvider oleSP = o as IOleServiceProvider;
System.Web.UI.Page oPage;

IntPtr pObject;
Guid sid = typeof(IVSMDDesigner).GUID;
Guid iid = typeof(IVSMDDesigner).GUID;
int hr = oleSP.QueryService(ref sid, ref iid, out pObject);
NativeMethods.ThrowOnFailure(hr);

IDesignerHost designerHost = null;
if (pObject != IntPtr.Zero)
{
   try
   {
       designerHost = Marshal.GetObjectForIUnknown(pObject) as
IDesignerHost;
       oPage = (System.Web.UI.Page)designerHost.RootComponent;
   }
   finally
   {
       Marshal.Release(pObject);
   }
}

============================================================

Thanks for taking the time to reply Adam.

James