Hopefully this is just a temporary bug or error, as YouTube is still present on the Apple TV, but disappeared from the iPhone and iPad. On the other hand, it could be Apple trying to break away from having Google services on its device, as it introduced its own Maps app in iOS 6.
We'll keep you updated on this matter...
Update: Apple has commented on this matter to The Verge
Our license to include the YouTube app in iOS has ended, customers can use YouTube in the Safari browser and Google is working on a new YouTube app to be on the App Store.