Just to clarify to one of the replies above: there are a few other factors that contribute to this, not just the screen size itself: resolution and font size.

I suppose you can make a rough estimation of the pixel density of n900 from your experience with the n810, but then again the screen size difference is not that much. I actually raised the same concerns on the weeks leading to n900's release, but one of the senior members here whom had an early n900 loaner insisted that "you won't notice the screen difference (from n8x0).". Well, i beg to differ. I think he was experiencing new gadget euphoria and he didn't quite realize that he was holding the device closer to his eyes or something.

Font size was trickier to determine, since different apps may use different text size and we didn't have access to maemo5 to test.

Anyway, i share the OP's concerns on this matter.