Trivial etiquette question - can he touch my laptop screen?
mgrouchy: I agree completely, people do this all the time, waving pens at my screen, etc. I think the easiest way to do it was the way you suggested, most people will get the hint if you just polish the smudge they left in front of them. You don't have to be rude or anything. You can even be discreet about it. People will still get the point.