I think its a permanent thing for the sunday 4pm movie. So far. They may add more movies in widescreen if its successful.
The thing is most of Disney's main output (i.e. cartoons) are made in america and in 4:3. The only things at the moment that can be shown in widescreen are movies, and perhaps new episodes of art attack.
It's interesting that the sunday movies are shown in widescreen, as next weeks is Mary Poppins. Was this made in widescreen? Or was the remastered version done in widescreen? It says it is Dolby Sound, and widescreen, so i dont know!
Widescreen film production was becoming widespread in the late 50s/early 60s I think - as it was something that 4:3 (and previously in the UK 5:4) TV couldn't compete with. Similarly stereo audio has been around the cinema since Fantasia (which is pre-war...) It is therefore likely that Mary Poppins was widescreen, and possibly stereo, in the 60s. (Even if not stereo then it may have been remastered in stereo now)
As for most US productions being 4:3 - this is less and less the case - especially with the major networks. US HDTV production is in 16:9 - and increasing amounts of material on the main nets is being shot HD. This means material from ABC,CBS,NBC and Fox is now routinely shot 16:9 - and as ABC own Disney it may be that 16:9 stuff from ABC appears?