All this does is stream a video, not the entire desktop of the computer ? Or did I miss something?
It's meant for media streaming. Mostly Video. Like iTunes movies, Netflix, Youtube, etc.
But to answer your original question, I've seen apps that allow your iDevice (iPhone, iPod Touch, iPad) to serve as an extra display for your computer.
Being that the aTV2 uses the iOS operating system, I don't see why you wouldn't be able to download it and install it after you jailbreak the thing.
There are apps currently being developed for jailbroken aTV2s that seem really promising, like Plex and AirFlick. Plex, for example, allows you to stream pretty much any video format to your aTV. Try to do some research into those.
However, I think he most sensible solution would be a mac mini connected do your TV, that you could remotely control with bluetooth keyboard and mouse, or from another mac (maybe PCs too) via Screen Sharing.