BTSense - Sensor app for NXT/EV3/Arduino/Raspberry Pi
Your Android Smartphone has more than a dozen sensors.
With this app you can access those sensors through your programs on the NXT brick (and soon EV3 brick).
The brick communicates with this app using Bluetooth and demands sensor data. And the app streams the sensor data back to your brick. You would then use that sensor data in your program to do whatever you want.
The app will be free for most sensors and others will be available for purchase.