From the article:
However, AirPlay is a lot more open now than it used to be. App developers and online video sites have the option to incorporate AirPlay. Until recently, I didn’t even think of trying AirPlay with anything but iTunes-sanctioned content.
I started experimenting this weekend. With the excellent PBS iPad app, I was able to beam recent episodes of NOVA from my tablet to my Apple TV. It also works with streaming video sites like YouTube and music apps like Pandora.