Apple's trailers app says it supports HD in the description under Product Features when I pull it up in itunes. I haven't tried it on my iPad yet, but here is a link.
Does anyone know what the default youtube setting is - 720p or 1080p? I was watching some videos last night that looked great, but I'm not sure if they were being played in their full 1080p or not.
I went into the YouTube app on my 4S & searched for "4K". The first 3 videos to show up are some that I watched with my Mac. Even if the iPad can't display the full resolution, it should scale to the Pad's full display capability.