Responding to this from Laurie's post: "but hypertext still does change and those changes are perceivable and quantifiable..."
I think the idea that hypertext changes is exactly what Manovich is saying doesn't happen. My take: Manovich argues (pages 55-61) that clicking links in hypertext gives readers the illusion that they're following their own ideas when, in fact, they are following prescribed paths. (I'm here referring to self-contained hypertexts, not the Web.) Hypertexts externalize associational thinking--take processes that Freud saw in the mind and make them physical--but prescribe it in doing so. We no longer can do our own associational thinking, but instead follow the possibilities for it encoded in the text.
But perhaps I'm misunderstanding what you mean when you say hypertext changes. Can you expand on that some more?
Also, perhaps this is where "open" versus "closed" interactivity become important. Manovich mentions the concepts briefly at the top of page 56, but doesn't do much with them afterward. So another question (showing my game-theory naivete here): are these established concepts in vg theory and, if so, how do they relate?