Cell phone distracted driving has been proven by studies to be plenty dangerous. Easily much more dangerous than non threatening, unsolicited advice giving I'd say. Therefore as a society I believe it is our duty to stigmatize this behavior, like we have with drunk driving. Publicly calling out those who insist on putting us all in danger is a small step to help reach this goal.
Personally I favor doing the talking hand thing, or honking, while giving the yacker a serious stare down. The frightening thing is not the small chance that a driver will attempt to "teach me a lesson", (I've yet to have a person willing to drop their phone to come "get" me) but the large amount of drivers that don't even come out of their cell induced trance, even while being stared or honked at!
I would never tap or touch a vehicle though. Actually, I'm never really close enough to reach one, whether stopped or not.