If you don't know, your app will not be able to get a UUID anymore in iOS 5.
I am fixing this bug, but I see one thing: if I compile my app for iOS 4, and install the app in iOS 5, the unique identifier still work. My old programs have no problem in iOS 5.
So, can I say this to my boss with 100% certainty?
If I compile for iOS 4, my program will have the unique identifier for all iOS 5 devices! It will only fail when I compile my program for iOS 5 or Apple stops supporting iOS 4 (and this will take a long time)!