Make sure you're running the latest version of iTunes.
Download Seas0nPass: Mac [10.6 or later] |Windows [XP or later]
A micro USB cable will be required to connect Apple TV 2G to computer.
Disconnect all iPods, iPhones, and iPads from the computer before running Seas0nPass.
Extract the archive you downloaded. Then double click the extract application to launch it
You might be asked to sign in as an Administrator. Enter your username and password then click the OK button.
You will be asked to enable UI Scripting. Click the Enable button.
Click the Create IPSW button from the main screen.
Seas0nPass will now download the latest Apple TV firmware file
Seas0nPass will create a custom IPSW file for you.
Once your custom IPSW file has been created, Seas0nPass will ask you to connect the Apple TV to your computer via a micro-USB cable and use the remote to put it into DFU mode by holding MENU and PLAY/PAUSE for 7 seconds. The power cable should be unplugged during this process.
Once your Apple TV has been detected in DFU mode, Seas0nPass will upload some files, automatically launch iTunes, and beginning restoring to the new firmware.
That's it! Your AppleTV 2G is jailbroken!
Update: Here is a video tutorial for this jailbreak: