"a document-oriented system that never requires the user to launch or quit an application, nor to "save" their work, would still represent a leap forward today."
While saving work is thankfully disappearing as a paradigm in iOS (I don't use Lion, so don't know), it's still application-based, meaning that if there's a tool I like in one app, I don't have access to it unless that app is open, and I can't call the tool up on its own.
iOS is a major leap forward, but at its heart, it's still more suited to consumption than creation.
Have you looked at iOS or Mac OS X Lion?