How about an app you can choose to run if you're in a terrorist/robbery/hostage situation. That will allow the authorities to use your mic and cameras to get a picture of what is going on.
How about using interconnected sensors (around towns/large crowd areas) to detect when a vehicle's weight is abnormally large (car/truck bomb). Then alert authorities to the vehicle.
A type of bomb proof enclosed area you have to walk though to enter a stadium that sniffs for explosives and uses imaging to determine if you are a threat, you could be detained inside if wearing an explosive vest for example or have a gun. This would prevent bombers from being in a crowd when detected.
Keep thinking outside the box.
The techniques themselves may be useful in other domains where there aren't as many privacy implications.