Zooming in and out on iPhone

What is the theory behind the pinch zoom in and out technique used on the iPhone?? How would one do this? Do you get the distance between 2 touch points and measure them? can someone point me in the right direction :)

Thanks :P

Here is the end code, it works for zooming in and out as well as moving around the iPhone, good usage in IOS games

 //Author DOMINIC OBOJKOVITS
 var check : boolean = false;
 var OPinchDist : float;
 var currentDist : float;
 var counter : float ;
 var touchP : Vector2;
 var thing : Transform;
 var speed : float = 0.1;
 var Osize : float;
 
 function Start(){
     
     iPhoneKeyboard.autorotateToPortrait = false; 
     iPhoneKeyboard.autorotateToPortraitUpsideDown = false; 
     iPhoneKeyboard.autorotateToLandscapeRight = false; 
     iPhoneKeyboard.autorotateToLandscapeLeft = false;
 
 }
 
 function Update () {
     
     var count = Input.touchCount;
     
     //Limits
     Osize = Camera.main.orthographicSize;
     if (Osize >= 12){
         Osize = 12;    
     }
     if (Osize <= 4){
         Osize = 4;    
     } 
     speed = 0.05 * (Osize/5);
     
     //Zooming Factor
     if (count >= 2){
     
         if (check == false){
             OPinchDist = Vector2.Distance(Input.GetTouch(0).position,Input.GetTouch(1).position);
             check = true;
         }
         currentDist = Vector2.Distance(Input.GetTouch(0).position,Input.GetTouch(1).position);
         var zoomFactor = ( (OPinchDist) / (currentDist) );
     
         Camera.main.orthographicSize = Osize * zoomFactor;
 
     }
     if (count <= 1){
         check = false;    
     }
     
     //Moves the camera around
      if (Input.touchCount == 1 && 
       Input.GetTouch(0).phase == TouchPhase.Moved) {
         var touchDeltaPosition:Vector2 = Input.GetTouch(0).deltaPosition;
         thing.Translate (-touchDeltaPosition.x * speed,-touchDeltaPosition.y * speed, 0);
     }
     
     
 }