Some time ago, I remember Microsoft having made a video for a "courier tablet" that was essentially an electronic notebook. Either side of the spine was, by default, just plain "paper" that could be drawn or written on. It had some kind of intelligence to recognize stuff, so if you started drawing out a grid it had basic capabilities that could be attached to that grid, like spreadsheets.

At least, these are things I think I remember. It's not quite what you're imagining, but it was cool and along the same lines.