According to an alleged email sent from Steve Jobs, we will be seeing AirPlay move over to Safari and 3rd party apps sometime in 2011.
The question was asked by a user of iOS 4.2 on both the iPad and iPhone 4 who asked if the ability to use AirPlay on the said apps would be possible. Another brief but positive answer came back as seen in the email below.
As it stands right now, AirPlay does work on Safari and third-party apps although it only streams audio. Next year we could be seeing the video streams also integrated so that users can send video direct to their Apple TV.
Another problem was pointed out by Daring Fireball who mentions that video shot on the iPhone 4 cannot be sent to the Apple TV...
The most obvious shortcoming: You can't use AirPlay to play videos shot on your iPhone. That's an obvious feature, right? Shoot a video on your iPhone, then play it back for family and friends on your big TV via AirPlay. No syncing, no docking, no moving files around. Just hit play. But it doesn't work in iOS 4.2.1 - even though you can use AirPlay to show still photos from the Photos app.
Perhaps the iOS 4.3 update will address this. Rumors indicated that 4.3 could be launching in the next couple of weeks although it seems now that it wont be till next year sometime.