But that doesn't stop avoidable artifacts from showing up, such as end-users who run their HDTVs at a "full screen" mode, which distorts the presentation of SD material. While it is possible that your local PBS station is doing something wrong, professionals typically know what they are doing. And PBS has (our) money to hire capable engineers.
I'd just like to add a wrinkle here that messes up what people see at home. The big cable stations like FX, TBS and TNT are offered in SD and HD versions on my cable dial (Verizon FiOS).
When they show an "old-style, 20th century" (as Spock would say) program, it looks fine, with black pillars along the sides, on the SD channel that nobody watches. But simultaneously on the HD channel that everybody watches, they stretch the image horizontally to fill the wide screen.
So you can have your flat screen TV set up correctly and still see absolute junk-- a picture that gives me eyestrain and nausea. If you are unaware of the SD alternate channel and try to fix the picture with your remote, you can get it nearly right, but the people become a little too tall and thin. It's just wrong enough to still be unwatchable.