The picture is stretched to fit the screen. Look for a setting to change the aspect ratio to normal(your screen is 16:9, the picture is 4:3) - either in the tv menu or the cable box (or maybe both). You will have black bars on each side of the picture when it is right - unless you are watching an HD channel which will fill the entire screen undistorted. Sorry for the jumbled response.
It's funny, this happened on my dad's new 650. He was messing around with it and couldn't get it straight. What happened was the cable box was set to 480p stretched... soon as I adjusted it to 1080i he was like, "holy hell that's a great picture!"