While previously announced that AirPlay allows for the redirection of any media from an iOS device to another Airplay enabled iOS device, it appears that AirPlay also supports drawing any content within the framebuffer out to a remote display, just as the current video-out functionality within iOS Applications on the iPhone or iPad.
As such, it is possible to develop applications for the Mac as well as iOS to stream the output of any application to the AppleTV for display on your HDTV, as long as the output is rendered prior to being sent across the network to the display device.
Practical applications for this are using the AppleTV as a wireless video card for your computer (serving as an additional monitor), presenting documents wirelessly to a display, or running games remotely.
While it is possible to redirect the output of any application to the AppleTV via a plist hack on jailbroken devices, the output is limited to the resolution as outputted by the application in the landscape-left orientation. It is always possible that one may find a work-around to this issue, however odds are that the tweaks will need to be made by the Application Developer themselves, much as the UIDeviceFamily tweak introduced issues for running iPhone applications on the iPad at native resolution.