Sequencing actions for tutorial purposes as well as the ability to recreate problems from sequences of user actions are two other applications of this idea. But as I said, we’re only at the conceptual stage. No precise decisions about how it will work have been made yet. I just thought I’d mention what my thoughts are about this type of problem.
As to Eric’s question, my guess is that there would be a single "untitled" undo history, so the moment you create a new document, you’d probably lose the history for the previously unsaved document.