Like this, but with third-party apps
It's being reported that Apple has already started approving third-party applications with AirPlay video support.
The iOS 4.3 update will open up the ability for those Apps to beam AirPlay to Apple TV. Currently, with iOS 4.2 AirPlay video can only be shared through Apple's native iOS video player, which severely limits the potential of the technology.
We've already seen a few tricks for getting AirPlay video to share across devices, such as this Totem plugin for Linux computers. Then there is AirView, which lets you share content between iOS devices or AirPlayer for Mac, which will share video with a computer running OS X.