Here's a patch (based on gtk+-2.8.6) for the cell renderer combo that will add a
"selected" signal when the renderer doesn't have an entry. The "selected" signal
is emitted when the user chooses an item in the popup menu and sends the path in
the tree view store and an iterator to the cell renderer's store (the one that
defines the popup menu). This allows the application to access more data from
the renderer's model. A small test application is also supplied.

---

--- gtkcellrenderercombo.h.orig	2005-12-16 10:25:13.918172000 -0600
+++ gtkcellrenderercombo.h	2005-12-16 10:33:50.476049000 -0600
@@ -51,6 +51,10 @@
 struct _GtkCellRendererComboClass
 {
   GtkCellRendererTextClass parent;
+
+  void (* selected) (GtkCellRendererCombo *cell_renderer_combo,
+	  const gchar	*path,
+	  GtkTreeIter	*riter);
 };
 
 GType            gtk_cell_renderer_combo_get_type (void) G_GNUC_CONST;
--- gtkcellrenderercombo.c.orig	2005-12-16 10:25:07.121590000 -0600
+++ gtkcellrenderercombo.c	2005-12-16 10:30:48.842937000 -0600
@@ -20,6 +20,7 @@
 #include <config.h>
 #include <string.h>
 
+#include "gtkmarshalers.h"
 #include "gtkintl.h"
 #include "gtkbin.h"
 #include "gtkentry.h"
@@ -59,6 +60,13 @@
   PROP_HAS_ENTRY
 };
 
+enum {
+	SELECTED,
+	LAST_SIGNAL
+};
+
+static guint combo_cell_renderer_signals[LAST_SIGNAL];
+
 #define GTK_CELL_RENDERER_COMBO_PATH "gtk-cell-renderer-combo-path"
 
 G_DEFINE_TYPE (GtkCellRendererCombo, gtk_cell_renderer_combo,
GTK_TYPE_CELL_RENDERER_TEXT);
@@ -130,6 +138,16 @@
 							 TRUE,
 							 GTK_PARAM_READWRITE));
 
+  combo_cell_renderer_signals[SELECTED] =
+	  g_signal_new("selected",
+		  G_OBJECT_CLASS_TYPE(object_class),
+		  G_SIGNAL_RUN_LAST,
+		  G_STRUCT_OFFSET (GtkCellRendererComboClass, selected),
+		  NULL, NULL,
+		  _gtk_marshal_VOID__STRING_BOXED,
+		  G_TYPE_NONE, 2,
+		  G_TYPE_STRING,
+		  GTK_TYPE_TREE_ITER);
 }
 
 static void
@@ -270,6 +288,8 @@
   if (canceled)
     return;
 
+  path = g_object_get_data (G_OBJECT (combo), GTK_CELL_RENDERER_COMBO_PATH);
+
   if (GTK_IS_COMBO_BOX_ENTRY (combo))
     {
       entry = GTK_ENTRY (gtk_bin_get_child (GTK_BIN (combo)));
@@ -279,10 +299,12 @@
     {
       model = gtk_combo_box_get_model (GTK_COMBO_BOX (combo));
       if (gtk_combo_box_get_active_iter (GTK_COMBO_BOX (combo), &iter))
+      {
+	g_signal_emit( cell, combo_cell_renderer_signals[SELECTED], 0, path, &iter );
 	gtk_tree_model_get (model, &iter, cell->text_column, &new_text, -1);
+      }
     }
 
-  path = g_object_get_data (G_OBJECT (combo), GTK_CELL_RENDERER_COMBO_PATH);
   g_signal_emit_by_name (cell, "edited", path, new_text);
 
   g_free (new_text);
--- gtkmarshalers.list.orig	2005-12-16 10:27:32.741536000 -0600
+++ gtkmarshalers.list	2005-12-16 10:27:49.847567000 -0600
@@ -92,6 +92,7 @@
 VOID:POINTER,POINTER,POINTER
 VOID:POINTER,UINT
 VOID:STRING
+VOID:STRING,BOXED
 VOID:STRING,STRING
 VOID:STRING,INT,POINTER
 VOID:STRING,UINT,FLAGS

----

#include <gtk/gtk.h>

static void destroy( GtkWidget *widget, gpointer data )
{
    gtk_main_quit();
}

static selected( GtkCellRenderer *renderer, gchar *path,
   	GtkTreeIter *riter, gpointer user_data )
{
	GtkTreeModel *rstore;
	char *v1, *v2;
	GtkListStore *store = user_data;
	GtkTreeIter iter;

	g_object_get( renderer, "model", &rstore, NULL );

	gtk_tree_model_get( rstore, riter, 0, &v1, 1, &v2, -1 );

	if( gtk_tree_model_get_iter_from_string(GTK_TREE_MODEL(store),&iter,path) )
	{
		gtk_list_store_set( store, &iter, 0, v1, 1, v2, -1 );
	}
}

int main( int argc, char **argv )
{
    GtkWidget *window;
	GtkWidget *tree;
	GtkListStore *treestore, *store;
	GtkCellRenderer *renderer;
	GtkTreeIter iter;
	char *value;

    gtk_init( &argc, &argv );

    window = gtk_window_new( GTK_WINDOW_TOPLEVEL );

	treestore = gtk_list_store_new(2, G_TYPE_STRING, G_TYPE_STRING);
	gtk_list_store_insert_with_values( treestore, &iter, 0, 0, "0", 1, "A", -1);
	gtk_list_store_insert_with_values( treestore, &iter, 1, 0, "1", 1, "B", -1);
	tree = gtk_tree_view_new_with_model( GTK_TREE_MODEL(treestore) );

	store = gtk_list_store_new(2, G_TYPE_STRING, G_TYPE_STRING);
	gtk_list_store_insert_with_values( store, &iter, 0, 0, "0", 1, "A", -1 );
	gtk_list_store_insert_with_values( store, &iter, 1, 0, "1", 1, "B", -1 );
	gtk_list_store_insert_with_values( store, &iter, 2, 0, "2", 1, "C", -1 );
	renderer = gtk_cell_renderer_combo_new();
	g_object_set( renderer, "model", store, "text-column", 0,
	   	"has-entry", 0,
	   	"editable", 1, NULL );
	g_signal_connect( renderer, "selected", G_CALLBACK(selected), treestore );

	gtk_tree_view_append_column( GTK_TREE_VIEW(tree),
		gtk_tree_view_column_new_with_attributes("Index",
			renderer, "text", 0, NULL) );

	gtk_tree_view_append_column( GTK_TREE_VIEW(tree),
		gtk_tree_view_column_new_with_attributes("Value",
			gtk_cell_renderer_text_new(), "text", 1, NULL) );

	gtk_container_add( GTK_CONTAINER(window), tree );

    g_signal_connect( G_OBJECT(window), "delete_event",
		G_CALLBACK(destroy), NULL);

    gtk_widget_show_all(window);

    gtk_main();

    return 0;
}