This thread is really fascinating, ranging from the hilarious to the intelligent. I agree that touching isn't THAT big of a deal that you need to get all worked up. However, there are legitimate reasons for not touching people's screens. I have a screen with a matte display -- that's without the extremely glossy and reflective glass covering. If someone has even the shortest of short fingernails -- and most people do -- they're going to do some kind of damage to the rather sensitive, uncovered screen. I'm not about to go get a mirror -- other people call them glossy screens -- just to protect my screen from your projectile fingers. If I do touch a screen, which happens because like some person said, you don't always think about it, I make sure to touch with the pad of my finger and not the nail. And pens are a complete nofuckingway no matter what kind of screen it is. They will ruin any screen. You ever try to get ink out of a tiny indentation in a glass screen? Because you can't. It's happened to me.

So in conclusion, if your screen isn't glass-covered and they're coming at it like an ICBM, definitely say something polite (Excuse me..., Please don't..., Umm...). But if your screen is protected and they're not using a pen/knife to point...let it go. Most screens are so bright you can't even see smudges.