I've got one on my N810 and notice absolutely no difference. As someone who is really obsessive compulsive about keeping electronics in tip top shape, I can't imagine owning any more gadgets without full body protectors. My buddy has one on his Palm Pre and I didn't notice any difference, even with multi-touch.

Zagg is very very likely to come out with a shield for the N900. They've got shields for some of the crappiest phones and odd devices. I don't see why they would not make one. It just takes a bit of time until the N900 is more saturated in the US.