This works fine. Thx a lot!
You have to obtain a path object and work on that, viz:
Gtk::TreeModel::Path tree_path(curr_iter);
tree_path.prev(); // tree_path now refers to the row of the list store
// preceding the selected one
Gtk::TreeModel::iterator prev_row_iter = [list_store]->get_iter(tree_path);
[list_store] is a Glib::RefPtr<Gtk::ListStore> reference to your list store
(if that's what you are using).