Widescreen Mode was created in 2003.

American Widescreen Museum was created in 1997.

Widescreen wallpaper is meant to be a wallpaper for a widescreen monitor. It is usually in high definition to showcase the quality of the widescreen monitor. Regular wallpaper has a different resolution and size. If displayed on a widescreen, regular wallpaper will have two black bars on the left and right hand side.

Yes, a widescreen TV is still measured diagonally.

Widescreen has been used in movies since the late 19th century. However, widescreen TVs were not available until July of 2000.