YouTube now provides High Quality video playback option on Windows Phone

The YouTube web app now offers access to most of the features of desktop YouTube, except for uploading and downloading videos.While the browser application remains at v1.0, the actual streaming of the videos via HTML5 has improved to the point that you now have the option of watching some videos in HQ. Windows Phone users also have available some third party YouTube players, like YouTube Pro

Via WPCentral