XBMC has finally been released for the new Apple TV. Here you have all the instructions you need to get it working on your device.
NOTE: the following HOW-TO requires you to have Jailbroken ATV2 version 4.1 or 4.2.1.
Here is how to jailbreak your ATV2:
How to jailbreak Apple TV 2 on iOS 4.2.1 with Seas0nPass
How-To: Jailbreak Apple TV 2G on iOS 4.2.1 with PwnageTool