- (void) viewDidLoad {
  ...
  if (![self.navigationController isNavigationBarHidden])
    [self.navigationController setNavigationBarHidden:YES animated:NO];
  ...
}

- (IBAction) pushAnotherView:(id)sender {
  AnotherViewController *anotherViewController = [[AnotherViewController alloc] initWithNibName:@"AnotherViewController" bundle:nil];
  [self.navigationController pushViewController:anotherViewController animated:YES];
  if ([self.navigationController isNavigationBarHidden])
    [self.navigationController setNavigationBarHidden:NO animated:YES];
  [anotherViewController release];
}

- (void) viewWillAppear:(BOOL)animated {
  if (![self.navigationController isNavigationBarHidden])
    [self.navigationController setNavigationBarHidden:YES animated:YES];
  [super viewWillAppear:animated];
}