the support is implemented as part of libgnomedb, which installs a
libglade module containing the support for the GNOME-DB widgets. This
is the correct way, AFAIK, as does libgnomeui, IIRC. So, it seems that
module is not being found by libglade, or maybe it's got some stuff
missing in it.

Could you see if the module is being installed and where?

Also, could a libglade hacker please check this?

I have the modules

$ rpm -ql libgnomedb|grep glade
/usr/lib/libglade/2.0/libgnomedb.a
/usr/lib/libglade/2.0/libgnomedb.la
/usr/lib/libglade/2.0/libgnomedb.so

The widget is displayed at runtime, but I cannot access it from Mono.
Here's the code (posted on gnome-db mailing list)
=== 
Glade.XML gui;
GnomeDb.Grid db_grid;
Gtk.VBox vbox_main;
 
gui = new Glade.XML( null, "../gui/test.glade", "application", null );
gui.Autoconnect( this );
db_grid = (GnomeDb.Grid)gui.GetWidget( "db_grid" ) ;
 
//this works
vbox_main = (Gtk.VBox)gui.GetWidget( "vbox_main" ) ;
=== 
Error is:
Unhandled Exception: System.InvalidCastException: Cannot cast from
source type to destination type
 
This code is interesting:
Gtk.Widget w;
w = ui.GetWidget( "db_grid" );
Console.WriteLine( w.ToString() );

it prints "Gtk.VBox"

I'm looking into this. Any suggestions welcomed.

Looked into glade-gnome.c from libgnomeui, they implement almost every
widget there, but on gnomedb they are generic. 
libgnomeui works fine from mono with libglade.

I tried this in glade-gnomedb.c:
glade_register_widget (GNOME_DB_TYPE_GRID, grid_build,
grid_build_children,  NULL );

static void
grid_build_children (GladeXML *xml, GtkWidget *parent,
		    GladeWidgetInfo *info)
{
    int i;

    for (i = 0; i < info->n_children; i++) {
	GladeChildInfo *cinfo;
	GtkWidget *child;

	cinfo = &info->children[i];

	if (cinfo->internal_child) {
		glade_xml_handle_internal_child (xml, parent, cinfo);
	} else {
	    child = glade_xml_build_widget (xml, cinfo->child);
	}
    }
}

static GtkWidget *
grid_build (GladeXML *xml, GType widget_type,
	   GladeWidgetInfo *info)
{
    GtkWidget *grid;

    grid = glade_standard_build_widget (xml, widget_type, info);

    return GNOME_DB_GRID( grid );
}

But same result. 

Frédéric Logier said on mailing list it works for him in C, but I
don't have a full test-case.

Rodrigo, any ideas?
note: I think this bug should be moved to gnomedb now, maybe later to
gtk-sharp.

This sounds like a GTK# bug, if it still occurs at all.  Most likely it hasn't
registered its GnomeDB wrappers, so is returing a wrapper for the closest
ancestor it can find (a GtkVBox in this case).