The YouTube app that comes pre-loaded with most iOS devices was made by Apple and hasn't been updated in ages and Apple decided to ditch it altogether in iOS 6. Users who still want access to YouTube as a standalone app can now go and get Google's official App for iPhone and iPod Touch. The new app actually offers more content since the older YouTube app didn't play ad-supported videos.
Seeing the brilliant engineering Google has employed in recent iOS apps (have you seen Google + on the iPad? Stunning!) we expect an iPad optimized version that's Retinized to come along shortly.