After weeks of work involving several geeks, Appletvhacks released a patch allowing Apple TV owners to use an external USB HD as a primary storage unit.
Once the patch installed, the Apple TV will be able to use the largest USB HD enclosure currently available on the market, so at least 1 TB!. Keep in mind that the patch was written and tested only with Apple TV software version 1.0, while version 1.1 is currently the last one available.
[translation by Linathael]