Wired has noted that "Soon, thanks to the tireless efforts of the iPhone Dev Team, you will be able to install apps on your AppleTV. An upcoming Jailbreak tool, called SHAtter, has already been used to unlock the new Apple TV's firmware."
The units are supposedly now shipping. I have a lot of questions about the device (like how will it handle the photo screen saver if your local machines are offline) but hacking it might make the thing more usable (divx please, and how about letting my screen share my desktop to my TV?).