A close look at the Info.plist property list for Lowtide.app, the iOS app that runs Apple TV, suggest that the app could be headed to the iPad in some form. There a property with two "UIDeviceFamily" entries - one for the Apple TV and one for the iPad - which indicates that the app could run, unmodified, on either device. The hooks aren't there in the mainstream iOS 4.1 or the 4.2 beta, but a future OS release - or some clever hackery - could provide them.
The UIDeviceFamily property is an array that contains two elements:
UIDeviceFamily = ( 3, 2 );
The implications here are interesting. Lowtide relies on AirPlay to streaming audio and video to it. The iPad doesn't support AirPlay - yet - though in a tweet from @AriX he announced that he got the AppleTV's AirPlay daemon to run on his iPhone 4, allowing his to stream iTunes audio to his phone. And as we know from Nick's report this morning, @dhowett got Lowtide to run on an ipt4.
AirPlay everywhere opens up really interesting possibilities all around, from streaming of video content on multiple devices, to streaming apps and other data to the Springboardless Apple TV. And if AirPlay - which is, after all, just a UDP streaming protocol based on RTSP - could be modified to work over 3G, well, the sky is the limit, isn't it?