richms: I disagree that a 16:9 letterbox is the correct choice for analog, the correct is to give the 4:3 center cut and anything that is not 4:3 safe, well tough, people can upgrade to digital to get ratio selection if they have an older television. Without obvious problems like that then there is little incentive for people to upgrade.
There are pros and cons to both methods.
I personally think letterbox is a better option simply because it removes the need for networks (and content producers) having to factor in 4:3 safe areas. With centrecut feeds for events such as live sport and news it means that onscreen GFX have to be put in 4:3 safe areas rather than the far left and right which means significant picture area is lost inside the 4:3 safe area. These GFX should be further to the outside of the picture so the viewer doesn't miss out on what is actually happening!