You should use the
- (void)didReceiveMemoryWarning
and
- (void)dealloc methods.
In iOS 6, the viewWillUnload and viewDidUnload methods of UIViewController are now deprecated. If you were using these methods to release data, use the didReceiveMemoryWarning method instead. You can also use this method to release references to the view controller’s view if it is not being used. You would need to test that the view is not in a window before doing this.
So you should check if your view is in the window first, then remove your observer in the didReceiveMemoryWarning