Uh oh. It looks like Apple has pulled Airplay from the YouTube application according to MacStories. That means it is available App by App and not universally by MP4 files (unless you keep older betas working on your iPad obviously).
Interestingly, Airplay works on YouTube via Safari but not on the YouTube App itself.
That means that app vendors can probably decide if their apps can be shot around the room to AppleTVs and other AirPlay devices.