Need Help. Vizio LCD not processing 1080 signal

Just purchased a new Vizio 47" sv470xvt (120hz) for my parents which is connected to a cablevision HD box. When I set the cablebox to 480 stretch, 720 or 1080i the picture does not fill the screen (black bars on left and right) and does not appear to be in HD. The only time the picture fils the screen is when I set the cablebox to 480 std. We watched a HD movie last night (88 minutes) via pay per view and the TV made the film look like it was shot on a home video camera - very unusual. This is my 3rd Vizio LCD and I have never experienced a problem like this before. Is it possible that the chip in the tv is simply broken?

Thanks for the reply.
1) I have tested across multiple HD channels
2) The TV is hooked up via HDMI from the HD cable box
3) It does have a zoom mode (Zoom and Wide) and is messed up in both modes

I will continue to try and find the problem today and if unsuccessful I will return to Sam's Club in the morning (2 hour drive) for the 52" Sharp LCD.

Thnaks for your help. Please let me know if you have any other ideas.

I was able to find Component cables and after conducting a test with them the results are the same. I did just test a DVD and everything seems fine which leads me to believe the problem might be with the cable box. I am going to test a different cable box on this tv

I was able to find the problem. CableVision here in NY (I live in TX) does not start their HD channels until station 700 so the channels I was testing were indeed all analog.

Thank you very much for offering help!