If I use the example shown at the bottom of this page:
http://developer.gnome.org/doc/API/2.2/gtk/gtk-migrating-GtkComboBox.html

which adds two renderers (pixbuf and text) and packs them into a layout, they
are aligned centered to the GtkComboBox.

This seems to occur after the GtkComboBox has been set up.  By that I mean, if
you use glade OR use the following code:

        combo_box = gtk_combo_box_new_text ();
        gtk_widget_show (combo_box);
        gtk_box_pack_start (GTK_BOX (hbox), combo_box, TRUE, TRUE, 0);
        gtk_combo_box_append_text (GTK_COMBO_BOX (combo_box), "Jabber");

then go to change the layout, this bug is demonstrated.

To fix this I have noticed that using the gtk_cell_layout_reorder() function
works if you do it for index 0 and 1.  But also using gtk_cell_layout_clear()
fixes this bug but with a notable error:

(test_gtkcombobox:5393): Gtk-CRITICAL **: file gtkcellview.c: line 707
(gtk_cell_view_cell_layout_clear_attributes): assertion `info != NULL' failed

After tracking this down further by compiling against GTK 2.4.10, it turns out
the renderer is NULL when calling gtk_cell_view_get_cell_info() on line 706, and
the function wouldn't match any of the renderers in the list for that cellview
anyway (because it is NULL) so perhaps a check is needed?