Appletvhacks published videos and photos demonstrating that they have successfully installed OSX on an Apple TV thanks to a patched kernel.
To achieve it, one has to extract the HD, perform a backup, then install and modify OSX. Then the hacked Apple TV can then support a keyboard and a mouse, and one can install most applications and software, taking into account the poor CPU capabilities and the small amount of RAM installed.
Despite proving that the Apple TV is nothing but a small Mac running OS X, installing the OS X on such a device is mostly useless as the 256MB of RAM will force the HD to swap all the time; making the machine hardly usable and much less responsive than with its dedicated original system.
[update]: disclaimer: As the announcement was released on April 1st, we can not entirely exclude that this hacking is an "April Fool".