Xdialog v1.0 was originally written by Alfred Reynolds at Cyberone Internet in September 1999.
I discovered this utility in April 2000. At the time being, it was very basic (fixed menu size and position, only the "dialog" widgets and the "treeview" were implemented and the "dialog" compatibility was less than perfect). But it was exactly what I was searching for and I then decided to improve it so to use it in a project I was developing at work.
Xdialog development has been taken over by me (Thierry Godefroy) from v1.1.0 ownwards (v1.1.0 was never released, the first public release by me was v1.2.0).
Some Xdialog users also contributed to this project: thanks must go to Albert Chin-A-Young who helped a lot in making possible to compile Xdialog on non-Linux plateforms, as well as to Luke Holden, Luis E. Limon, Ben Bauer, Barry Rowlingson, Taco R. de Vries, David B. Harris and Thanh Ma for their suggestions regarding Xdialog improvements, not to forget Sid Boyce and Thomas Ribbrock who helped to chase down bugs in early Xdialog releases (I hope I didn't forget anybody ! ;-).