That'd probably be the only way you couldn't tell easily how small the screen is compared to the n800. For example, my cousin's iPhone screen size looks similar to my tablet (especially when I'm not fullscreened and you can see how icons eat up space). But if he launches Youtube (an app that doesn't up the entire screen) or I go into full screen I can clearly see the difference in screen size. But for iPhone apps that use the entire screen there's no noticable difference (at least that one can easily see).

The problem with a 3.5 inch screen is that if the UI is still going be like Diablo (with the icons on the side and icons at the top) then a 3.5 screen will easily show the smaller real estate compared to the n800. But if the UI hides way like the iPhone then the smaller screen size won't be an issue.

Obviously since the device isn't finished yet you can't answer this question. But one problem with the n800 and n810 is the recessed screen (how the frame/case of the device is on top of the screen kinda). Which makes it hard to hit things like the X button in the upper right hand corner with fingers. I wonder if the next device will fix this. It also makes scrollsbars tough to use at times (even with a wider scrollbar theme).