Framed
From what I can see, it looks like most shows taped in widescreen for tv are not pan & scan for full screen. Instead, what happens is when they tape the show, the shots are set up so that all the important stuff happens within the traditional 4:3 view.
They shoot the show as though it were a fullscreen shot, and just chop off the sides when showing the sd version. In other words, they are not taking full advantage of the wider aspect ratio, since they stick everything smack dab in the middle. On regular shows this usually means that the sides are either unoccupied of filled with the back of someone's head.
Football is a great example of the framing problem. Since they have it set up for a sd shot, to fit all the main players on the left and right of the screen in the shot, they cannot zoom in as much and you end up with the top of the screen filled with the crowd. If they framed it for widescreen, they would be able to get closer to the action.
Despite the fact that sports is one of the things that pushes HD sets, this will be the last thing to go to a letterbox format. SD viewers, imo, would not stand for it, even as they begin to accept scripted programs that are letterboxed.