I claim this is a mistake by the channel provider, and there should be no such option as "overscan" for a digital TV. They (the channel provider) are converting the entire 525 line analog presentation into 480 digital, rather than just taking the picture portion which is already 480 scan lines IIRC.
It is not just the channel provider and is not an HD problem. It is also the TV manufacturers that set their TVs to overscan as the default. With pixel for pixel (the best setting) on a 1080p set you will see occasional artifacts from supplemental signals on the edge, but rarely on anything but ads. With a 720p set the Dish option is handy for reducing the overscan.