Jailbreak limera1n was released today for 4.1 iDevice users in what appears to be a contentious situation in the hacking community. Windows-only, the new jailbreak is not expected to work with Apple TV or debut on Linux and OS X for a bit.
The previously announced dev-team jailbreak, which was due out tomorrow, may be put on hold while the team works on revising their tools for the limera1n exploit rather than exposing multiple exploits that will quickly be patched.
Jailbreaking, while legal, is based on finding system weaknesses to allow hobbyists to inject code that will open their devices to full system read-write access. It is this full access that allows third party developers to customize the OS and install new applications and extensions.
Having looked forward to developing for Apple TV starting tomorrow, count me among the disappointed.
Update: The Limera1n page has updated to indicate that Apple TV jb is "technically supported". I can confirm that I put my ATV2 into DFU mode and was able to "jailbreak" it. But since no AFC services or OpenSSH were installed (let alone APT), there's nothing that really can be done with it at this time.