The format in which HD programs are sent out can not be controlled. This is normal. How you display the image on your HDTV can be controlled according to your perference. Most people who have widescreen HDTVs would simply set the aspect ratio to the wide(16x9) format for all of their viewing, at least I do. However, this does stretch the image on regular broadcast programs to fit the wide format. If this is not to your perference, you would need to switch back to the normal(4x3) format which would display the image with black bars on the side.
I'm shopping for new HDTV plasma, and within a number of reviews, I've seen the following statement, "cannot change aspect ratios with HDTV sources."
I know what aspect ratio is, but what does it mean if a television set cannot change aspect ratios? That it doesn't do it automatically, and needs to be set manually per TV channel or other source?