http://www.youtube.com/watch?v=zWJUJhmwapY&feature=player_embedded
As of the Apple TV 5.1 update released September 24, 2012, you can now rearrange apps that are not in the top row (Movies, TV shows, etc). Like the tap-hold gesture on an iPhone or iPad, simply hold the selection button of your Apple Remote on an app and it will start jiggling. Unlike an iPhone or iPad, the only app that jiggles is the one you’re about to move.

Arrow around to put the app where you like, and press the selection button again to confirm your choice.