I am also trying to unjailbreak my AppleTV (AppleTV 2 iOS 5.1). I am having trouble entering DFU mode. I had trouble when jailbreaking as well, however I forget how I finally got there. I think it was just repeated trying that finally worked but it is getting me nowhere now. As I understand it, I need to connect the AppleTV to my computer and hold down the menu and down button on the remote for around 6 or 7 seconds then release. I've also heard to try only releasing the down button and press the play button along with the continued pressed menu button for 6 or 7 seconds. Additionally I have tried simply pressing the Menu and Play buttons simultaneously for around 6 or 7 seconds. Unplugging and restarting all of these processes in different orders has not resulted in any success of the AppleTV appearing in iTunes. All of the attempts result in the AppleTv starting to blink semi rapidly as soon as I press down on the remote. It is an about twice per second blink as opposed to the about one per second blink scene immediately after plugging in, but not as fast as I've seen it get while jailbreaking, when it looks like it is almost solid because it is flashing so fast. Any suggestions on what I may be doing wrong?