title: UINavigationController not popping view on back button
body: <p>I have a UINavigationController.  I've got a UIViewController that I've pushed onto the stack using pushViewController.  The previous view controller has a backBarButtonItem simply titled "Cancel."</p>
  
  <p>While the new view animates in correctly, when I tap Cancel, the navigation bar animates as if the view was popped, but the new view doesn't go away.  Do I need to implement a delegate somewhere?</p>