It looks like the game is kinda ripped across the screen. I used to see it on my video card when I didn't have vsync enabled.
Mine goes horizontally about a third of the way up the screen and a third of hte way down.
I emailed microsoft and they want me to call in with the issue.
I have the factory VGA cable BTW.
From what I found out, it will go away if you lower your resolution, but that's not the answer I wanted to hear.
I've seen it on Burnout: Revenge, Dead Rising, and Gears of war. I'm running in 1080p too.
Plat is right about the vsync, just wanted to be sure I wasn't the only sumbitch that was seeing it.