The iPhone Dev-Team has been using a toolrunning iOS 4.1. It's not in userland yet--meaning, no, it's not ready for you to download and try--but it's a very promising jailbreak. It uses an exploit at a very low level of iOS that they say Apple might not be able to patch easily.
Shatter isn't just for iPhones and iPod Touches, it seems. A video recently posted on the Dev-Team's blog shows the exploit being applied to the new Apple TV. Right now, it doesn't do much; there aren't any apps written for the Apple TV yet.
But that's right now. The fact that (at least at launch) the Apple TV can be jailbroken is exciting for some (like me). It's a platform ripe for apps, and Apple may have that planned for the device, but Shatter, when released to the public, gives us the ability today.
Yes, I want Twitter in my living room. Don't judge.