The metaphor is only skin deep
One of the "new" apps that was made available for the iPod Touch is Notes. (Yes, I paid the $20 bounty on them rather than jailbreaking it. I just wasn't worth the drama.)
The app looks like a yellow legal pad. It uses a comic font to simulate handwriting (although my actual handwriting never looked as neat), it has hand-drawn icons for email and trash, and it plays a page turning animation when you switch from one note to another.
As I've said before, this kind of detail sells software. But there is one caveat. The metaphor should never limit the application.
In the early 90's, as software was becoming a commodity in the lives of average people, the graphical user interface was going mainstream. We saw several attempts to make software look like a real thing. The Macintosh and Windows both had desktop metaphors, some online "communities" actually looked like streets with buildings for different destinations, and of course there was Microsoft Bob.
As Alan Cooper tells us in the Myth of Metaphor, software should not limit itself based on the metaphor it chooses. It should be idioms to guide UI design. Idioms are patterns that users come to recognize, even though they have no basis in reality, and may not even be intuitive.
The iPod Touch employs several idioms that we were used to from previous iPods and other software. For example, you can scroll through a list, pick an item, and then the screen slides to the left to reveal a new list. When you go back, the prior list slides back in from the left. The Touch also created some new idioms thanks to its multi-touch screen. Now you can scroll through those lists with a flick of the finger. Most people have to be shown these idioms, but once they see them they apply them everywhere.
The iPod Touch Notes application is not a slave to its metaphor. For example, you can make a note as long as you like. It is not limited to the length of a piece of paper. If it strictly adhered to its metaphor, you would have to turn the page to keep writing. Thankfully, it's one page per note.
But the Notes application does adhere to the iPod Touch idioms. The notes appear in a list, which scrolls with a flick of the finger and slides to the left when you select a note. Notes themselves scroll, even though the "binder" stays fixed at the top of the screen. No physical legal pad in existence could work this way, but that doesn't keep us from understanding it.
The metaphor is only there to give the app a consistent visual design. The idiom gives it its usability.