My TV has a setting for this kind of thing. I have a Sony 30" Wega widescreen CRT tv.
I have mine set to show everything originally in 4:3 to show in 16:9. The setting for HD is set to OAR or original aspect ratio. Some shows and commercials will have the black bars on the sides, but will always go back to 16:9 if the show is in widescreen.
All SD is in 16:9.