I have hit a very teething problem with DMVPN. The issue is if I have two tunnels at the Hub and Spoke routers, and if I shutdown one tunnel at the Hub, the Spoke routers still think that tunnel is UP. This is true considering all tunnels are mGRE tunnels so as long as the physical interface remains UP, the tunnels will remain UP.
However, I am looking for a trick where the Spoke routers react to failure of a tunnel on Hub router. For a sample diagram, please use this URL. Please note the configuration on that page is incomplete.