Rotation Limit

If you want to rotate 180 degrees quickly (but not instantly) when the key is pressed, maybe a coroutine is the best solution. Care must be taken to not call the coroutine again while it's running, what can be done with the auxiliary boolean variable turningBack : it's set when the coroutine starts and cleared when it ends, and the code in Update must be blocked while turningBack is true. There's a pitfall however: rotating exactly 180 degrees is a problem because Unity may not identify about which axis you want to rotate (the object may rotate about very weird axes!) To avoid this problem, the coroutine splits the 180 degrees in two 90 degrees rotations done in sequence.

The code could be something like this:

 var turnBackTime: float = 0.5; // duration of turn back
 private turningBack = false; // true while turning back
 
 function TurnBack() // coroutine
 {
   turningBack = true; // don't call me - I'm turning back!
   var rot0 = transform.rotation; // initial rotation
   var rot1 = rot0 * Quaternion.Euler(0, 90, 0); // first 90 degrees
   var rot2 = rot1 * Quaternion.Euler(0, 90, 0); // last 90 degrees
   var rate: float = 2 / turnBackTime; // calculate the rate for 2 90 degrees rotations
   var t: float = 0; // control variable
   while (t < 1){ // rotate first 90 degrees
     t += rate * Time.deltaTime;
     transform.rotation = Quaternion.Slerp(rot0, rot1, t);
     yield;
   }
   t = 0; // do next 90 degrees rotation
   while (t < 1){
     t += rate * Time.deltaTime;
     transform.rotation = Quaternion.Slerp(rot1, rot2, t);
     yield;
   }
   turningBack = false; // rotation finished
 }
 
 function Update ()
 {
   // first change in Update: abort it while turning back
   if (turningBack) return;

   // the code below is the same as before:
   var controller : CharacterController = GetComponent(CharacterController);
   transform.Rotate(0, Input.GetAxis ("Horizontal") * rotateSpeed, 0);
   var forward = transform.TransformDirection(Vector3.forward);
   var back = transform.TransformDirection(Vector3.back);
   var curSpeed = walkSpeed * Input.GetAxis ("Vertical");
   controller.SimpleMove(forward * curSpeed);
   if (Input.GetAxis("Horizontal") < 0.0){
     animation.CrossFade("turn_left");
   }
   if (Input.GetAxis("Horizontal") > 0.2){
     animation.CrossFade("turn_right");
   }

   // here's the second modification in Update:
   if (Input.GetButtonDown("Run") && (Input.GetAxis("Vertical") < 0.0)){
     animation.CrossFade ("turn_back");
     walkSpeed = 0.0;
     TurnBack(); // start the coroutine
   }
   ...

why don`t you use the mecanim animation system?

take a look at Quaternion.Slerp and at transform.Rotate http://docs.unity3d.com/Documentation/ScriptReference/Transform.Rotate.html