How To Put Apple TV 2G In DFU Mode
Now that Apple TV 2G jailbreak is possible using Limera1n, many ATV users may want to try their hands at jailbreaking their brand new device. Currently there are no apps for Apple TV, so there is nothing much you can do after jailbreaking Apple TV 2G. But hey, in the world of jailbreak, you never know how things turn up!

In order to jailbreak Apple TV, you will have to put it in DFU mode. For the unaware, DFU mode is different from Recovery or Restore modes. In this mode, the screen goes complete blank just like it is powered down.
The iBoot and the OS are not yet loaded when your device is in DFU mode. This is when the boot level exploit used by Limera1n is patched onto your device, thus jailbreaking it. You can also downgrade or restore your device with a custom firmware in this mode.

Apple TV DFU Mode
iPhone hacker @StealthBravo has found the DFU mode on ATV and here are the steps:
1. Plugin your ATV into your Computer via microUSB.
2. Now reboot your ATV by holding down Menu + Down buttons together for around 6 seconds.
3. After reboot, immediately hold Menu + Play until you see the message in iTunes saying that Apple TV in recovery mode is detected.
Even if you don’t want to jailbreak now, go ahead and save SHSH blobs for Apple TV 2G using TinyUmbrella. This will help you downgrade when needed.

SBSettings Alternative for iPhone 4S/iPad 2, No Jailbreak Required
How to Unlock iPhone 4S on AT&T without Jailbreaking
Save SHSH Blobs of iOS 5.0.1 for iPhone 4S with TinyUmbrella
Gmail App for iPhone, iPad, iPod Touch Available for Download
Enable FaceTime over 3G in iOS 5 [Jailbreak Tweak]
iTunes 10.5.1 for Windows, Mac OS X [Direct Download Links]
Jailbreak iOS 5.0.1 with Redsn0w 0.9.9b8