The fact Apple TV runs Intel and Mac OSX under the bonnet means it has lots of hacking potential, while the fact it does so little - only playing nicely with iTunes and MPEG4 or H.264 - gives would-be hackers plenty of incentive to expand its feature set. Apple doesn't seem to have gone out of its way to stop people modifying the Apple TV. The case is easy to open, it runs on standard hardware and software and there's even a built-in factory restore if things go pear-shaped (hold down Menu and - on the remote).
With Apple limiting the Apple TV's functionality but making it relatively easy to hack, power users have the freedom to do as they please but Apple doesn't have to clean up the mess when things go horribly wrong. I'd call that a win, win situation. After trying out an Apple TV I wouldn't buy one today, but I might in the future if people can expand the feature set - especially if it's done via the USB port so you don't have to void your warranty by popping the bonnet.
So hack on, I say. Hopefully someone can transform the Apple TV into something non-Apple-worshippers would want to use.