TUAW's Erica Sadun, if ever a wiz there was, has once again hacked away at Apple's iOS 4.2 AirPlay feature and created an app called AirPlayer that gets it to stream video from iPhone or iPad to the Mac.
What AirPlayer does is create and advertise a custom Bonjour AirPlay service that pretends to be an Apple TV. Bonjour is Apple's zero configuration networking solution for allowing devices and applications to communicate with each other over local area networks. When Apple created AirPlay, it basically set up a new way for Apple TV to interact with iOS using Bonjour communications.
She's not a Windows dev but says in theory the same thing should be possible on that platform and perhaps Linux as well.
You don't need to Jailbreak to try this out (the app runs on your Mac, not your iPhone or iPad) but if you're not Jailbroken and running AirVideoEnabler you'll be limited to YouTube and iPod/Videos for your streaming (AirVideoEnabler lets 3rd party apps stream AirPlay video as well).
If you want to try it out, you can get AirPlayer from Erica's website, below. Video after the break!