I barely have to touch the screen on the n800 to have the input register, and that is with a screen protector.

I did play around briefly with the n900 at the NYC meetup, and did not really notice any issues with the touchscreen. I did not spend enough time to truly compare, but reputedly the n900 is supposed to be more responsive than the n800/n810. If that's the case, then I don't think there should be any worries about screen sensitivity. If you are used to a capacitive screen, that's probably another story though.