It seems like something's going on around the "AirPlay button working everywhere" so many people were excited about: specifically, we just noticed Apple removed AirPlay support in the native Youtube app on iOS 4.2 Beta 3.
(screenshot of iOS 4.2 beta 1)
We tested this on our iPad a few minutes ago: the AirPlay button is nowhere to be found in the Youtube app, but it's there if you open Youtube using Safari. The Youtube app used to have an AirPlay button in previous 4.2 betas, as reported by iSpazio, 9to5mac and AppAdvice. Apple basically implemented AirPlay in the default media player framework, so any app based on that would be able to automatically support AirPlay without any additional configuration needed.
Now, the removal of AirPlay in the Youtube app could be a bug (which would be very weird, considering all the bug fixes introduced in this new beta), or Apple's decision to limit the usage of AirPlay. I don't know why they would do that, though: maybe they don't want to piss off content providers such as Netflix and, yeah, Youtube by enabling AirPlay by default? If so, perhaps developers will need to manually enable AirPlay in future versions of iOS, even if the app is based on the default media player framework. We don't know much about this right now, we'll have to wait for updates in future versions of iOS 4.2.
Update: Some of you suggested that Apple might impose this restriction because they don't want users to stream video to an audio-only external device, such as an Airport station. Then why would apps like iFiles still be able to stream video to the Airport Express? I've personally tested that. For the sake of the argument, it'd make me think that it's not Apple that doesn't want Youtube videos to be streamed to an Airport station, but Google. Like I said, we're just speculating here. Could as well be a simple bug. A rather interesting one, though.
(AirPlay still working fine in the Spotify iPhone app)