A modified version of the SmoothLookAt script found on the Unity3D wiki: http://wiki.unity3d.com/index.php/SmoothLookAt_CS

SmoothLookAt.cs

using UnityEngine;
using System.Collections;

public class SmoothLookAt : MonoBehaviour {
    public Transform target;
    public float distance = 10.0f;
    public float height = 5.0f;
    
    //Animation curves to adjust the smoothing of
    //the transform's overall movement
    public AnimationCurve rotationDampingCurve;
    public AnimationCurve heightDampingCurve;
    
    //The overall speeds at which the above curves
    //happen, 0.5 takes 2 seconds to complete
    //the above curves, 2.0 takes half a second
    //to complete the above curves
    public float rotationDampingSpeed = 1.0f;
    public float heightDampingSpeed = 1.0f;

    void Start () {
        Debug.Assert(target != null);
    }
    
    void LateUpdate () {
        // Calculate the current rotation angles
        float targetRotationAngle = target.eulerAngles.y;
        float targetHeight = target.position.y + height;

        float currentRotationAngle = transform.eulerAngles.y;
        float currentHeight = transform.position.y;

        // Calculate the damping amounts
        float diffAngle = Quaternion.Angle(target.rotation, transform.rotation);
        float rotationDamping = rotationDampingCurve.Evaluate(diffAngle);
        float heightDamping = Mathf.Clamp01(Mathf.Abs(targetHeight - currentHeight));
        heightDamping = heightDampingCurve.Evaluate(heightDamping);

        // Damp the rotation around the y-axis
        currentRotationAngle = Mathf.LerpAngle(currentRotationAngle, targetRotationAngle, rotationDamping * (Time.deltaTime * rotationDampingSpeed));

        // Damp the height
        currentHeight = Mathf.Lerp(currentHeight, targetHeight, heightDamping * (Time.deltaTime * heightDampingSpeed));

        // Convert the angle into a rotation
        Quaternion currentRotation = Quaternion.Euler(0, currentRotationAngle, 0);

        // Set the position of the camera on the x-z plane to:
        // distance meters behind the target
        transform.position = target.position;
        transform.position -= currentRotation * Vector3.forward * distance;

        // Set the height of the camera
        transform.position = new Vector3(transform.position.x, currentHeight, transform.position.z);

        // Always look at the target
        transform.LookAt(target);
    }
}