Boomerang

Indeed - the 4:3 broadcast that I saw on Disney Channel tonight was an episode of Hannah Montana from 2008.

Didn't Turner Classic Movies only go widescreen when their HD channel launched a couple of years back? I think their policy has always been to show films in their original aspect ratio, which meant some of the ultra-widescreen films from the 1950s looked particularly bad in 4:3. (Incidentally I've noticed that they've shifted from showing 'classic' movies from that time period, to a mix of 1970s Westerns in the daytime and modern classics at night).

Prior to HD, many super widescreen films (the 2:35:1 I think it's called) were quite often aired in 4:3 Pan-and-Scan (not just on TCM but other broadcasters as well), with most of the issues that causes.

If nothing else the transition to HD means we don't have to put up with horrible Pan and Scan anymore.

Weren't some cartoons made in widescreen formats for their original cinema showings? Are they ever shown this way today?

The last 20 or so episodes of the original Tom & Jerry cartoons were widescreen/CinemaScope but when aired on Boomerang are pan & scan 4:3.

I'm happy to be proved wrong but I can't find any evidence that any of the well known cartoon shorts (as they were, being only about 7 or 8 minutes long) were ever made in widescreen prior to about 1953. Disney's first CinemaScope cartoon dates from 1953, and MGM's first CinemaScope Tom & Jerry cartoon is from 1956 - I don't believe Looney Tunes ever went Cinemascope, and it ran for nearly 40 years first time round.