I've never seen it done with anything else in American TV.
Cartoon Network does it, they show everything in widescreen including older cartoons. In the early days of HDTV, AMC was even worse as they'd stretch their already cropped pan-and-scan versions of movies. As a somewhat early adopter of HDTV it was more prevalent than it is now. Of course, now there's less and less 4:3 programming being shown, period.
Allyn Gibson wrote:
My parents have to adjust their HDTV manually. There's something like four different settings on their television -- letterboxed all around, letterboxed on the sides, stretched, cropped. (The cropped one is weird -- it stretches the 4:3 to the 16:9 width and then lops off the top and bottom.) They usually have me fix it when I visit, because they do something like get the television stuck on the cropped setting.
My TV has a 4:3 mode which will take those streched-out broadcasts and force them back into 4:3 which is pretty handy. I take it that isn't a common feature.
I've also found a good number of people who are oblivious to their TV stretching out the image and a good number that aren't aware they are watching the SD channel instead of the HD one (when both are available).