iOS5でviewDidAppear:内でaddChildViewController:するとChildViewControllerのviewWillAppear:が呼ばれない

なにこれ……

AppDelegate.h

//
//  AppDelegate.m
//  SimpleNestedViewController

#import "AppDelegate.h"

@interface ChildViewController : UIViewController
@end

@implementation ChildViewController

- (void)viewWillAppear:(BOOL)animated
{
  [super viewWillAppear:animated];

  NSLog(@"iOS5だとviewWillAppear:が呼ばれない");
}

@end

@interface ParentViewController : UIViewController
@end

@implementation ParentViewController

- (void)viewDidAppear:(BOOL)animated
{
  [super viewDidAppear:animated];

  ChildViewController* childViewController = [ChildViewController new];
  [self addChildViewController:childViewController];
  [self.view addSubview:childViewController.view];
  [childViewController didMoveToParentViewController:self];

  dispatch_async(dispatch_get_main_queue(), ^{
    // ここに移動するとNSLog呼ばれる
  });
}

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
  self.window.rootViewController = [ParentViewController new];
  [self.window makeKeyAndVisible];
  return YES;
}

@end

(iOS SDK 7.1 / iOS 5.1.1 で確認)