Unity3D: Follow and object - Source: http://wiki.dreamsteep.com/Unity_ai

FollowObject.js

var waypoint: Transform;
var speed: float = 15; 

function Update () {

   var target:Vector3 = waypoint.position;
   var moveDirection: Vector3 = target -transform.position;
   
   var velocity = rigidbody.velocity;
   
   if (moveDirection.magnitude <1)
   {
     velocity = Vector3.zero;
   }else{
    velocity = moveDirection.normalized*speed;
   }

  rigidbody.velocity = velocity;

}