Follow after height

 var playerHeight : float;
 var player : Transform;
 var raiseCameraMinHeight : float;
 var raiseSpeed : float = 5.0;
 var baseHeight : float;
 
 function Start()
 {
 if(!player)
 player = GameObject.FindWithTag("player");
 }
 
 function Update ()
 {
 playerHeight = player.position.y;
 if(playerHeight > raiseCameraMinHeight)
 {
 var targetHeight = (playerHeight - raiseCameraMinHeight);
 transform.position.y = Mathf.Lerp(transform.position.y, baseHeight + targetHeight, Time.deltaTime * raiseSpeed);
 }
 else
 transform.position.y = Mathf.Lerp(transform.position.y, baseHeight, Time.deltaTime * raiseSpeed);
 }

//think this should do it, be sure to set the cameraRaiseMinHeight var to the desired height to start raising, and the baseHeight to the lowest height the camera should go .. and tag your player as "player".. Oh yeah, this script would go on the camera, hope this helps