Its already been hacked. The AppleTV is just iOS but even more cut down. The only application on there is the "Frontrow" type application called Lowtide. People have transferred it on to other devices and it runs on there. Third party applications can already run on it as well just nobody has done much to support it.

I'm not sure where you're thinking these "decent" mods are though. People hacked the original Apple TV and that ran a cut down x86 OSX so porting was even easier but the additions were generally very clunky and not very good. If you thought Boxee was bad then look at some of the Apple TV 1st generation stuff :P

You can even run Boxee on the old Apple TV. Its just not very good because of the again high CPU consumption which makes the unit overhead and start crashing once you play video.