In one of my earlier blogs I had indicated how a smart phone application that is connected to my car can provide additional options for safety of the car against theft. The idea is that I can optionally switch on a feature that prevents the car's engine to start unless the smart phone is in close proximity. Optional because there will be valid cases when my smart phone may not be around like having given my car on loan to a friend. The same feature can be an add-on to the keyless entry that some of the cars support today.
The kind of applications that we can build with smart phones and the sensors all around us will only increase with time.