Firecore has released a jailbreak tool, Seas0nPass for Apple TV 2. This tool lets you to create custom firmware that can be restored in iTunes. This process resembles PwnageTool, for iPod Touch and iPhone.

How to Jailbreak iOS 4.2.1 on Apple TV 2 for Mac OS X
1. Download Seas0nPass.
2. Install iTunes. Make sure that you must have the latest version. For checking latest version, you can go to Apple menu and click Software Update.
3. Unzip Seas0npass.zip and launch it.
4. Select ‘Create IPSW‘.

5. Latest IPSW files to create custom jailbroken software file would be downloaded.

6. You will get a prompt to connect Apple TV using micro-USB cable. Do this, also leave power disconnected and hold both ‘MENU‘ and ‘PLAY/PAUSE‘ buttons for seven seconds.
7. Your AppleTV will enter in DFU mode.

8. iTunes will open automatically and will start the restore process.

9. iTunes will confirm the completion of restore process.

Congrats, your Apple TV is now jailbroken.
Complete info at Firecore.