The latest version of the iPhone Dev Team's PwnageTool, v4.1, has been released, with the new software updated to include jailbreaking of the new Apple TV. The hack is an untethered jailbreak, taking advantage of the limera1n exploit among other things, though once you've actually got a jailbroken Apple TV there's not a huge amount you can do with it right now.
"Right now, about all you can do is command-line stuff via ssh" the Dev Team explains, "you also have afc2 available, so you can use tools like ifunbox to move files around." The next step is waiting for the iOS developer community to jump on board and start playing around with the new-found access; then hopefully we'll see custom apps and more.
Meanwhile PwnageTool 4.1 also works on the iPad (firmware 3.2.2), iPod touch 4G, iPod touch 3G, iPhone 4, iPhone 3GS and iPhone 3G. It's available to download here.