from the API docs:
void        gtk_list_store_insert           (GtkListStore *list_store,
                                             GtkTreeIter *iter,
                                             gint position);

Creates a new row at position. iter will be changed to point to this new
row. If position is larger than the number of rows on the list, then the
new row will be appended to the list. The row will be empty before this
function is called. To fill in values, you need to call
gtk_list_store_set() or gtk_list_store_set_value().

when I do this with a value greater than the number of rows, I get:
(nnMedia:10511): Gtk-WARNING **:
gtkliststore.c:1073:gtk_list_store_insert(): position 40 is off the end of
the list

and 
    gtk_list_store_set (list, &iter,
			  NAME_STRING, gtk_entry_get_text(GTK_ENTRY(Name)) ,
			  FREQUENCY_INT, atoi(gtk_entry_get_text(GTK_ENTRY(Frequency))),
			  GROUP_STRING, gtk_entry_get_text(GTK_ENTRY(Group)),
			  INDEX_INT, atoi(gtk_entry_get_text(GTK_ENTRY(Position))),
			  -1);

doesn't do anything.

It works properly if the position is within the list.