Shake Gesture detection in UIWindow subclass

Question

I setup a UIWindow subclass as detailed in an answer from this post: http://stackoverflow.com/questions/150446/how-do-i-detect-when-someone-shakes-an-iphone/1351486#1351486

I am running into an issue where only the methods motionBegan: and motionCancelled: will fire. I need to detect the motionEnded: method, and whatever I do it simply will NOT fire on the device. It works fine in Simulator when using the "Shake Gesture" menu item. I have tried on two iPhones, and neither of them will fire the motionEnded: method.

I have tried to run the GLPaint sample code, and that works fine. I have also implemented the code in a UIViewController, and then all the methods work fine. I'd just like to implement it as an NSNotification instead of dealing with making a UIViewController first responder.

What could be the issue?