I got a Notepod+ (http://notepod.net/) for this very purpose - gives a sense of the size/basic feel of the thing and provides a nice place to sketch prototypes.

First thoughts:

- it's slightly smaller than I'd expected

- the screen, though, seems large because when you're wanting to look at something closely you naturally bring the "device" closer to your face... you don't do this nearly so much with a traditional notebook

- whoa, that's a lot of screen space to fill up... I need boundaries!

- I really wish this expensive notepad could surf the web, that'd be awesome.

yeah that's a good start... but there still in no substitute for actually manipulating the UI with your fingers instead of a mouse and spending some time with the real device.