There is something intrinsically “right” about seeing the iPad as a technological successor to, or version of, these physical objects. We’re immediately ready to accept the one as a substitute or enhancement for the other. This is a powerful, and novel, position for the iPad software developer.
It wasn’t true with the iPhone or iPod Touch; the devices are too physically small. The “Notes” app on the iPhone will forever be a simulation of a legal pad; the similar app on the iPad is a legal pad (to your user). It’s an incredibly important distinction in terms of how it influences our design.