A regular on MacRumors posted an email response on the lack of Airplay in Safari (and other 3rd party apps)
Currently, you need a hack from Cydia to enable Airplay for video, it is possible to make any iOS 4 device an speaker via Airplay...
Question: Hi, I recently updated both my iPhone 4 and iPad to 4.2. I think my favourite feature is airplay. This is seriously amazing and makes sharing content seamless. I just purchased Apple TV and was wondering are you ever going to make airplay video work for videos in safari and 3rd party apps? I hope to get a response.
Steve Jobs: Yep, hope to add these features to Airplay in 2011.