Continuous wire to monitor the proximity sensor

How would I go about implementing a thread that runs throughout the application. I have looked at services but I am pretty sure this isnt what I want. I dont want the thread constantly checking even if the application is closed. I just want a thread running in the background of the application (even if I switch between activities) to continously check to see if the user has raised the phone to his/her ear. If the user does then it will perform an action. Any examples of something like this?

You don't need a thread or service for this. See the example in the Android API documentation for the SensorManager. Also, see below for an example:

 public class SensorActivity extends Activity, implements SensorEventListener {
     private final SensorManager mSensorManager;
     private final Sensor mSensor;

     public SensorActivity() {
         mSensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
         mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_PROXIMITY);
     }

     protected void onResume() {
         super.onResume();
         mSensorManager.registerListener(this, mSensor, SensorManager.SENSOR_DELAY_NORMAL);
     }

     protected void onPause() {
         super.onPause();
         mSensorManager.unregisterListener(this);
     }

     public void onAccuracyChanged(Sensor sensor, int accuracy) {
     }

     public void onSensorChanged(SensorEvent event) {
         // TODO: implement your action here.
     }
 }