Player rotation affecting Top Down 2D Movement

As I'm attempting to control my character, from the perspective of the camera, when S is pressed, the character -always- moves directly south. However, the W,A,D keys move the character in a general north, east or west direction, respectively, As the player rotates via the mouse, the exact direct and speed changes.

Heres'a video of the behavior: http://www.youtube.com/watch?v=RosabgyZ2CE

And here's my code:

 #pragma strict
 
 var speed : float = 20.0;
 private var moveDirection : Vector3 = Vector3.zero;
 
 function Update () {
     
     var controller : CharacterController = GetComponent(CharacterController);
     
      moveDirection = Vector3(Input.GetAxis("Horizontal"),Input.GetAxis("Vertical"), 0);
     moveDirection = Camera.main.transform.TransformDirection(moveDirection);
     moveDirection *= speed;
       
     // Move the controller
     controller.Move(moveDirection * Time.deltaTime);
     
     var position = Input.mousePosition;    
     var newPosition = Vector3(position.x, position.y, Camera.main.transform.position.y - transform.position.y);
     var lastPosition = Camera.main.ScreenToWorldPoint(newPosition);
     transform.LookAt(lastPosition);
     
 }
 
 @script RequireComponent(CharacterController)

Any suggestions or help would be greatly appreciated.