When I asked the question, I had just purchased/installed the box and wanted to watch some HD shows, so I didn't bother reading the user manual. I now have.
There is a quick set-up procedure that verifies which settings your tv can display (480p stretched is one of those modes) along with a setting that lets you choose if the image should automatically be stretched to fill a widescreen tv.
Thank you all for the info about the 4:3 shows on the hd channels. One thing I do find odd, is that even 4:3 shows on the hd channels, the audio is 5.1.