[bookwormAT]

Aza Raskin discusses the feature in 2007: http://alistapart.com/article/neveruseawarning . I don't know if there is more/better material on the topic.

Personally, I have a very positive memory of when I discovered this feature in the early days of Gmail.

Edit: Guide van Rossum also wrote about this in 2005: http://www.artima.com/weblogs/viewpost.jsp?thread=97581

[jasomill]

Aza Raskin's father Jef incorporated the idea in his design for the Canon Cat, released in 1987. Notably, he felt Undo was such an important operation that it deserved its own large key[1]. He discussed many ideas behind the design, including universal undo as an alternative to confirmation, in his book The Humane Interface[2].

[1] http://en.wikipedia.org/wiki/File:Canon_Cat_keyboard.jpg [2] http://books.google.com/books?id=D39vjmLfO3kC&q=undo