In OSX Lion, Apple released Auto Save and Versions, which does exactly this. Except that just a tiny amount of applications I use in my daily development life does this. But in casual use, it's actually quite nice. If the application implements the whole set of Lion auto saving / state API's, you don't even have to save the files for them to remain available.

I can write on multiple unsaved documents with TextEdit, close TextEdit, and all the documents open up in their unsaved state as I open TextEdit. It's quite nice. But to have these features, requires too much of a workflow change to what people are used to, and the LaLa Land of it breaks down right after the user hits an application that does not implement the system, and loses their data (by clicking on don't save or assuming that everything is recovered after a crash).

More info on Auto Save and Versions: http://support.apple.com/kb/HT4753